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Principles of Physiological Psychology
By Wilhelm Wundt (1902)
Translated by Edward Bradford Titchener (1904)
[p. 241] CHAPTER VI
The Physiological Function of the Central Parts
§ 1. Methods of Functional Analysis
EVEN if we knew the course and the interconnexions of all the paths of nervous conduction, there would still be one thing needful for an understanding of the physiological function of the central parts: a knowledge of the influence exerted upon the processes of innervation by the central substance. And there is but one possible way of determining this influence: we must attempt to ascertain the function of the central parts by means of direct observation.
Under this limitation, two roads are open to the investigator who would gain an insight into the complicated functions of the nerve centres. He can arrange the phenomena in order of their physiological significance; or, accepting the lines of division drawn by the anatomists, he can examine into the separate function of each individual central region. It is obvious that the former of these procedures is to be preferred: not only because it lays the chief emphasis upon the physiological point of view, but also because, when the investigation of the conduction paths is over and done with, a doubt must still remain whether every one of the principal parts distinguished by anatomy represents a similarly well-defined functional area. In the present state of our knowledge, however, it is impossible to carry out the physiological programme with any sort of completeness. The method can be applied, with any hope of success, only to the two lowest central organs, myel and oblongata, where the phenomena may be referred without exception to two basal physiological functions, reflex and automatic excitations: the latter oftentimes deriving directly from nutritive influences exercised by the blood. We may well suppose that these same two basal functions are the source of the physiological activities of the higher central parts. At the same time, the interrelation of the phenomena is here so complicated, and their interpretation in many cases so uncertain, that it seems wiser, for the present, to examine each individual central area for itself with a view of discovering its physiological properties. We shall, accordingly, preface our enquiry by a general discussion of reflex and automatic action, in the course of which we shall have opportunity fully to consider the functions of the lower central regions; and we shall [p. 242] then proceed to investigate the brain and its parts in regular sequence from below upwards. We may, however, omit structures which, like pons, crura and corona, are intended in the main simply for the conduction of processes of innervation, and have therefore been sufficiently dealt with in the preceding Chapter.
The methods employed in the, functional examination of the central organs are, in general, the same as those which find application in the study of the conduction paths, save that anatomical investigation, which there holds the first place, must naturally play a merely subordinate part now that we are concerned with the activities of the organs. We shall ask assistance, where possible in combination, from physiological experiment and from pathological observation; and we shall pay attention, under both headings, to symptoms of stimulation and symptoms of abrogation. The special conditions of the phenomena are such that stimulation experiments must for the most part be employed in the general study of the reflexes and of automatic excitations, whereas the functional analysis of the various departments of the brain must rely almost exclusively upon symptoms of abrogation.
§ 2. Reflex Functions
(a) -- Spinal Reflexes
The simplest mode of central function, and the mode that still approximates most nearly to a simple conduction of processes of stimulation, is the reflex movement. In so far as the reflex process is a special form of conduction, we have discussed it in the preceding Chapter. But it is more; it is a form of conduction modified in various ways by the influence of the central substance. In the first place, the reflexes are not, like the processes of stimulation, conducted in both directions in the nerve fibres, but only in the one direction, from sensory to motor path: a fact which, as we explained above, is in all probability connected with the twofold mode of origin of the nervous processes in the motor cells. Secondly, the reflexes clearly show, in their dependence upon the stimuli that release them, the effects of the peculiar conditions of excitability obtaining in the grey substance. Stimuli that are weak and of brief duration fail, as a rule, to evoke a reflex movement: but the movement, once it appears, may far surpass in intensity and duration the direct muscular contraction set up by the same stimulus. Lastly, the central character of these processes is evinced by the dependence of the reflex centres upon other central areas with which they stand in connexion. Thus, it has been observed that the reflex excitability of the myel is enhanced by removal of the brain.[p. 243] It appears, therefore, that inhibitory influences are continually proceeding from the higher central organs, and lessen the irritability of the lower lying reflex centres. These are, in general, still more strongly inhibited if other sensory central parts, with which they are connected, are stimulated along with them. The reflex, e.g., released by excitation of a sensory myelic root or of its peripheral radiation is inhibited by simultaneous stimulation of the dorsal myelic columns, of quadrigemina and thalami, of another sensory root, or finally of peripheral organs within which sensory nerves are distributed. It is not improbable that the influence of the cerebral hemispheres belongs to the same group of phenomena; for this too proceeds, in all likelihood, from the terminations of the sensory conduction paths in the cortex. It has been observed that the inhibition of reflexes in mammals is especially strong if stimuli are applied directly to the centrosensory areas of the cerebral cortex. The mechanism of reflex inhibition would thus appear to be the same throughout: reflexes are inhibited, when the sensory cells that should transfer their excitation to motor cells are simultaneously excited with a certain degree of intensity from other sensory areas. This inhibitory effect is, however, limited to the condition that the areas whose stimulations interfere lie at a sufficient distance from one another in space. If adjacent sensory parts, or the nerve paths corresponding to them, are stimulated, the result resembles that of summation of stimulations within the same sensory area: that is to say, the interference gives rise not to inhibition but to intensification of the excitatory processes. Lastly, reflex excitations may also suffer inhibition from central elements interpolated in their own proper path. This is the interpretation of the fact that stimuli applied to the cutaneous radiations of the sensory nerves are more effective than stimuli applied to the nerve trunks, and that contrariwise the nerve roots become more irritable after their passage through the spinal ganglion. We must suppose, that is, that the subdivision of fibres in the sense organ serves, on the one hand, to increase irritability, and that, on the other, the excitation which arrives at a spinal ganglion cell there undergoes a certain inhibition: a combination of circumstances which, naturally, brings it about that the nerve trunk possesses a relative minimum of reflex excitability. The general lines to be followed, in an attempt to explain all these phenomena, are laid down by the principles of nerve mechanics that govern the reciprocal relations of excitatory and inhibitory effects, and by that morphological differentiation of the central elements which, in all probability, runs parallel with them.[p. 244]
There are, further, many phenomena which show, still in accordance with the general principles of nervous activity, that the individual reflex excitation aroused by a sensory stimulus does not by any means come upon the scene as interrupting a state of absolute non-excitation in the nervous elements. On the contrary, the state which we term the 'state of rest' is really a state of oscillation -- as a rule, of oscillation about a certain position of equilibrium -- in which the excitatory and inhibitory forces counteract one another. It is a state in which, on the average, there is a slight preponderance of permanent excitation, though this may be transformed, under special conditions, and more particularly under the influence of antagonistic effects, into a preponderance of permanent inhibition. In this way, the single transient reflex process is superinduced upon a reflex tonus, whose effects become apparent whenever there is interruption of the sensory paths in which the permanent innervation of reflex excitation is conducted. Thus, transsection of the sensory roots of an extremity is followed, in animals, by an atonic, quasi-paralytic state, which however neither abrogates the influence of voluntary impulses upon the atonic muscles nor prevents their action by way of concomitant movements. As regards intensity and distribution, these tonic reflex excitations appear, further, to stand under the regulative influence of all the manifold conditions imposed upon the organs by the nature of their functions. Reflex stimuli, which release transient reflex movements, may accordingly produce radically different results, according to the state of the pre-existing tonus and of the relative distribution of excitatory and inhibitory forces. Thus SHERRINGTON found, in observations upon animals whose myel was cut through in the cervical region, that extensor reflexes appeared if the leg was in the position of flexion, and flexor reflexes, if it was extended. We may also appeal to this influence of the variable conditions of permanent tonic excitation upon the individual reflex movement for explanation of the fact that the law of diffusion of reflexes with increasing intensity of stimulus (discussed above, p 162) admits of exception; the general and relatively constant conditions of reflex conduction are cut across by the more variable influences arising from the reciprocal regulation of sensations and movements.
(b) -- Metencephalic (Oblongata) and Mesencephalic Reflexes
The reflexes that have their seat in the oblongata are, in general, of a more complicated character than the spinal reflexes. This organ is, in particular, the seat of a number of compound reflexes, which play an important part in various physiological functions. We may mention the [p. 245] movements of inspiration and expiration, with the closely related processes of coughing, sneezing, vomiting; the muscular changes involved in the act of swallowing; the mimetic movements; vascular innervation and the movements of the heart. Many of these reflexes stand in an intimate relation of interdependence, as is indicated by the fact that their peripheral paths are oftentimes laid down in the same nerve trunks. Some of the above-named processes, such as the movements of respiration and the heart beat, result from a plurality of causes, and therefore continue after interruption of the reflex paths; in such cases, the reflex is but one of several determinants, and its influence is correspondingly restricted. Others, again, like the movements of swallowing, appear to be pure reflexes; they are abrogated by interruption of the sensory conduction to the reflex centre, even though the motor conduction to the muscles governing the movement have been left intact. All these reflexes alike, however, differ from the spinal reflexes on the point that, as a general rule, their sensory stimuli pass at once to a large number of motor paths. Many of them are essentially bilateral, and do not require the action of strong stimuli for their extension from the one to the other side of the body. Thus the respiratory movements, which are released by excitation of the pulmonary radiation of the tenth cranial nerve, involve motor roots that issue on both sides from the oblongata and from the cervical and thoracic portions of the myel. These movements furnish, at the same time, an illustration of a self-regulating reflex, which contains within it the impulse to continued rhythmical repetition. While the collapse of the lungs in expiration serves reflexly to start the movement of inspiration, their distention with inspiration serves conversely to excite the muscles of expiration. If the reflex impulse given to the expiratory muscles in inspiration is too weak to bring them into active exercise, it simply inhibits the antagonistic inspiratory muscles. This is the case in ordinary quiet breathing, in which inspiration alone, and not expiration, is connected with active muscular exertion. In the movements of swallowing, the regular sequence is apparently maintained by a different mode of self-regulation. The act of swallowing consists of movements of the larynx, pharynx and oesophagus: movements that succeed one another in regular order, on the application of a stimulus to the mucous membrane of the soft palate. The succession of movements in this instance is, perhaps, regulated in the way that stimulation of the soft palate releases, first of all, simply the movement of the palatal muscles, and that this in turn acts as a stimulus for the reflex elevation of the larynx and contraction of the pharyngeal muscles. In fine, then, it is probable that all these oblongata reflexes, whose detailed description belongs, of course, to physiology, are characterised by the combination of movements for the attainment of definite effects, -- the manner of combination being determined, oftentimes, by a [p. 246] mechanism of self-regulation, itself conditioned upon the reciprocal relation of a number of reflex mechanisms. A second noteworthy property of these reflexes is the following. The motor path of a given reflex movement sometimes stands in connexion with a second sensory path, from which, accordingly, the same movement may be aroused. Secondary sensory paths of this kind are connected, in particular, with the respiratory centres, so that the combined activity of the muscles of respiration becomes available for other purposes than those of inflation and emptying of the lungs. There is, e.g., a connexion of the sensory nerves of larynx and oesophageal mucous membrane (i.e. of the superior and, in part, of the inferior laryngeal nerves), and of the branches of the fifth cranial nerve distributed to the nose, with the centre of expiration. Stimulation of these sensory areas produces, first, inhibition of inspiration and then violent expiration. The latter is, however, preceded by a strong inspiration, the immediate consequence of the establishment of inhibition, due to the persistence of the influence of automatic excitation which we discuss below. Coughing and sneezing are, accordingly, reflexes of expiration; but they are not excited from the sensory area of pulmonary radiation of the vagus, from which the impulse to expiration ordinarily proceeds. They are distinguished by the fact that stimulation of the nasal branches of the trigeminus arouses not only the respiratory muscles but also the motor nerve of the face, the facialis, to reflex activity. The sneezing reflex consequently affords a direct transition to the mimetic reflexes of laughing, crying, sobbing, etc., in which again the muscles of the face unite in conjoint function with the muscles of respiration. Further: the secondary sensory path from the expiratory centre to the mucous membrane of the air passages is paralleled by a similar path from the inspiratory centre to the cutaneous investment of the body. We are thus able to account for the movements of inspiration produced by intensive stimulation, especially cold stimulation, of the cutaneous surface.
It may, then, be taken as a matter of common occurrence that in the oblongata a given motor reflex path is connected with a number of different sensory paths. But more than this: one and the same sensory path may enter into connexion, conversely, with several reflex centres, so that its stimulation arouses coincidently various kinds of reflex movements. Here belong, e.g., the mimetic reflexes, mentioned above, in which movements of respiration are combined with facial movements. A similar arrangement of connexions is in part responsible for the interaction of respiratory movements and heart beat. The heart is supplied by two sorts of nerve paths, which affect the sequence of beats in opposite ways: accelerating nerves,[p. 247] which increase the rapidity of heart beat, and inhibitory nerves, which diminish it, or bring the organ to a complete standstill. Both may be reflexly excited; but the centre for the accelerating fibres is intimately connected with certain sensory paths that run to the heart in the spinal nerves for the last cervical and first thoracic ganglion of the sympathetic; and the centre for the inhibitory fibres with others, that take their course for the most part in the cardiac branches of the vagus. Hence stimulation of the great majority of sensory nerves, and in particular of the cutaneous, laryngeal and intestinal nerves, produces inhibition, and stimulation of the sensory fibres that enter the muscles produces acceleration of the heart beat: this latter fact explains the increased action of the heart that accompanies general muscular exertion. Similar results follow from the movements of the lungs: their inflation accelerates, their collapse reduces the frequency of heart beat. The respiratory movements are therefore regularly accompanied by fluctuations of the pulse, whose rapidity increases in inspiration and decreases in expiration. On the whole, that is, the movement of the blood is accelerated by enhancement of the movements of respiration. Again, we find the same kind of interaction between the reflex connexions of cardiac and vascular innervation. The vessels are governed, like the heart, by motor and inhibitory nerves, both of which may be reflexly excited. Stimulation of most of the sensory nerves releases the motor reflex, i.e., acts upon the nerve fibres which constrict the small arterial blood vessels and thus produce an increase of blood pressure in the larger arteries, and which are therefore termed presser fibres. The only exception to this action is in the vessels of the part of the skin to which the stimulus is applied: these vessels usually dilate, either immediately or after a brief stage of constriction, and thus occasion the hyperæmia and redness of the stimulated parts. There are, however, various sensory areas which stand, conversely, in direct reflex connexion with the inhibitory or depressor fibres of the blood vessels, and whose stimulation leads accordingly to a widespread dilatation of the smaller vessels. Here belong, in particular, certain fibres of the vagus, which radiate within the heart itself and form its sensory nerve supply: fibres which, in all probability, are exclusively devoted to this reflexly mediated interaction between cardiac and vascular innervation. For their stimulation must be effected, in the normal course of physiological function, by increased action of the heart; this, in turn, is produced by increase of blood pressure and of the amount of blood contained in the arterial system; and this, once more, can be compensated only by a dilatation of the small arteries, which permits the outflow of the blood into the veins, and thus at the same time reduces the arterial blood pressure. We see, in fine, that all these reflexes of the oblongata stand in relations of interdependence, such that the functions discharged by this central organ mutually regulate and [p. 248] support one another. An intensive cold stimulus applied to the surface of the skin produces, reflexly, a spasm of inspiration and an arrest of heart beat. But the danger which thus threatens the life of the organism is avoided, since the expansion of the lungs serves, again reflexly, to excite expiration and acceleration of cardiac movement; while at the same time the stimulation of the skin brings about, by way of yet another reflex, a constriction of the smaller arteries, and so prevents any excessive emptying of the arrested heart. In many of these cases, as in a certain number of the reflexes proceeding from the myel, the central transmissions have simply a regulatory significance. The peripheral organs are the seat of direct innervation effects, due perhaps to special ganglion cells lying within them, perhaps to the excitomotor properties of the muscle fibres themselves; and the addition of the system of spinal and oblongata reflexes can do no more than modify these effects by way of excitation or of inhibition.
In all probability it is the nerve nidi of the oblongata, with their intercurrent central fibres, that we must consider as the principal reflex centres of this organ. The complicated character of the metencephalic reflexes appears to find its sufficient explanation in the anatomical conditions of these nerve nidi. They are, upon the whole, more strictly isolated than are the centres of origin of the spinal nerves. But, as an offset, certain nidi are closely connected by special central fibres both with one another and with continuations of the myelic columns. These two facts, taken together, explain the relative independence and singleness of aim of the oblongata reflexes. Myelic fibres are involved in these reflexes to a very considerable extent; and it is therefore probable that they are brought together, first of all, in some cinereal formation, and only after leaving this enter into connexion with the nerve nidi to which they are assigned. Thus, the respiratory motor fibres are, perhaps, collected in a special ganglionic nidus, which stands in connexion with the nidus of the vagus nerve. We may fairly suppose that this significance attaches to several of the grey masses scattered in the reticular substance. On the other hand, it is not probable that movements so complicated as the mimetic movements, or the movements of respiration and swallowing, possess each a single ganglionic nidus as their special reflex centre. Apart from the fact that centres of this sort, for complicated reflexes, have never been demonstrated, their existence is negatived by the nature of the movements themselves. The respiratory movements, e.g., evidently require us to posit two reflex centres, the one for inspiration, the other for expiration. Various mimetic reflexes, like laughing and crying, can he much more easily explained on [p. 249] the assumption of a reflex connexion, joining certain sensory paths at one and the same time with the respiratory centres and with determinate parts of the nidus of the facialis, than on that of an especial auxiliary ganglion, serving directly to initiate the above complex group of movements. In the same way, the movements of swallowing must be derived, like the respiratory movements, from the principle of self-regulation; we must suppose that the first movement of the entire process gives, as it is made, the reflex stimulus to the second, this the stimulus to the next following, and so on.
Of the four 'specific' sensory stimuli, two only are concerned, to any great extent, in the arousal of reflexes by way of sensory nerves: impressions of taste, and light stimuli. The former stand in reflex relation to the mimetic movements of expression; to reflexes, i.e., some of which (as we remarked above) readily combine with reflex respiratory movements, and thus lead us to infer a close connexion of the corresponding reflex centres. Light stimuli regularly evoke a twofold reflex response: first, closure of the eyelid, with a direction of the two eyes inward and upward, and secondly contraction of the pupil. Both reflexes are bilateral, though with weak excitations the movement is more pronounced upon the stimulated side. The reflexes released by way of the auditory and olfactory nerves appear in the neighbourhood of the external sense organs; if the stimuli are extensive, appropriate movements of the head may also be induced. In man, the proximate auditory reflexes are for the most part confined to contractions of the tenser tympani, which presumably accompany every sound stimulation: but in many animals reflex movements of the external ear are clearly observable.
If the stimulus is extensive, or the degree of irritability unusually high, the sphere of reflex activity may be extended beyond the limits of the direct reflex connexions. This phenomenon of diffusion is more definite and uniform for the cranial than it is for the spinal nerves. In the case of the optic nerve, e.g., the reflex to the muscles that move the eye-ball is connected, in extensive stimulation, with contraction of the corresponding muscles for movement of the head; and the facialis reflex to the orbicularis palpebrarum may be accompanied by concomitant movements of the othes mimetic muscles of the face. Reflexes touched off from the gustatory nerve fibres may cover a wider territory; they are apt to involve, not only the facial nerves, but the vagus centre as well. Stimulation of the sensory nerves of respiration is confined, as a rule, within the limits of its original reflex area. The strongest excitation of the central trunks of the pulmonary branch of the vagus has no reflex effect beyond the tetanus of inspiration. The reflex connexions of the expiratory fibres are more far-reaching. Stimulation of the sensory laryngeal nerves, and especially of their peripheral ends, is [p. 250] likely to involve the muscles of the face and of the upper extremity. But the fullest and most extended reflex relations are those of the trigeminus, the largest of the sensory cranial nerves. Stimulation of the trigeminus affects, first of all, its own motor root, which supplies the masseter muscles; and passes from this to the nerves of the face, the respiratory nerves, and finally to the whole muscular system of the body. There are two evident reasons for this range of reflex effect. First and generally, the trigeminus controls the largest sensory surface of all the sensory nerves, so that its nerve nidi also occupy a wide area, and opportunity is thus given for manifold connexions with motor centres of origin. Secondly and particularly, the position of its nidi is favourable. The superior nidi are situated, above the oblongata proper, in the pons; i.e. in the organ in which the ascending columns of alba are grouped together, by the interpolation of cinerea, to form the various bundles of the crus. We see, therefore, why it is that lesion of the oblongata and pons in the neighbourhood of the nidi of the fifth cranial nerve is followed by general reflex spasms. The result need not, of course, be attributed solely to these nidi: the stimulation in such cases may affect other sensory roots of the oblongata as well.
(c) -- Purposiveness of the Reflexes. Extent of Reflex Phenomena
The reflex phenomena bear upon them the mark of purposiveness. As regards the oblongata reflexes, this characteristic appears at once from the above description of their conditions and of their orderly co-operation. But the spinal reflexes also show, for the most part, a certain degree of the same quality. Thus, if a stimulus be applied to the skin, the animal makes a movement of arm or leg that is obviously directed upon the removal of the stimulus. If the reflex becomes stronger, the arm or leg of the opposite side will make a similar movement, or the animal will jump away, apparently to escape the action of the stimulus. Only when the movements take on a convulsive character, as they do with extremely intensive stimuli or in states of unusual excitability, do they lose this expression of purposiveness. These facts have suggested the question whether the reflexes may properly he regarded as mechanical consequences of stimulation and of its diffusion in the central organ, or whether they are actions of a psychical kind, and as such presuppose, like voluntary movements, a certain amount of consciousness. Worded in this way, however, the question is evidently misleading. There can be no doubt that the arrangements in the central organ can produce purposive results with mechanical necessity; we have the same phenomenon in any perfected form of self-regulating machinery. Moreover, the oblongata reflexes are highly purposive, and nevertheless [p. 251] dependent upon definite mechanical conditions Again, there is no reason whatever why a sensory stimulus should not release a reflex movement and arouse a sensation or idea at one and the same time: so that we cannot take the absence of all conscious process as the direct criterion of a reflex movement. On the other hand, the definition of the reflex would, it is true, be indefinitely extended, and the term would cover practically the whole range of organic movement, were we to apply it to any and every movement released in the central organ by the action of sensory stimuli. Suppose, e.g., that I make a voluntary movement, in order to grasp some object that I see before me. this, which is indubitably an act of will, still falls under the general heading of a movement released by sensory stimulation. It lacks, however, and conspicuously lacks, an attribute which is specifically characteristic of the reflex; the attribute, indeed, that first gave rise to the distinction between reflex and voluntary action, and without which the distinction loses all meaning. A movement mediated in the central organ by way of response to sensory stimulation, if it is to be denominated a reflex movement, may not bear upon it the marks of psychical causation; i.e., the idea aroused by the stimulus may not constitute, for the agent's own consciousness, the motive to the external movement. My involuntary reaction to a sensed stimulation of the skin is, therefore, a reflex, so long as the sensation remains a mere accidental concomitant of the movement, so long, that is, as the movement would be made in precisely the same way without such a concomitant sensation. On the other hand, reaction is not a reflex, if I voluntarily put out my hand to seize the stimulating object that is pressing upon the skin; for in this instance the movement is conditioned, for the agent, upon the conscious process. In the individual case it may, naturally, be difficult to decide, especially if the observations are made from the outside, whether a given movement is or is not a reflex. But this practical difficulty does not justify our setting aside altogether the criterion that distinguishes the reflexes from other forms of action, and leaving out of account the fact that, while related by their purposiveness to psychically conditioned movements, they differ from them, clearly and definitely, in the lack of conscious intermediaries. It is precisely this criterion that makes the reflexes an easily distinguishable and characteristic class of organic movements. We may also mention a further aspect of reflex action, closely connected with the criterion just discussed, though naturally of less universal application: the fact that reflexes follow immediately upon the operation of sensory stimuli, while psychically conditioned movements admit of a longer or shorter interval between stimulus and movement. What holds of this holds also of other objective characteristics, as e.g. that of the possibility of choice between different means. Such criteria are not always applicable: partly, because [p. 252] the characteristics do not attach at all generally to psychically mediated movements, but partly, too, because the purposive nature of the reflexes leaves a certain amount of room for difference of interpretation.
If we admit that these criteria are adequate to the empirical delimitation of the reflexes, as a readily distinguishable group of organic movements, we must also accept the conclusion that the central reflex area, in man and in the higher animals that resemble him, probably does not extend higher up than the mesencephalic region. In all cases where a sensory stimulus is conducted to the cerebral cortex, and there for the first time transformed into a motor impulse, the central transference appears, without exception, to involve the interpolation of psychophysical intermediaries; so that the action is presented to the agent's own consciousness as psychically conditioned. Many authors, it is true, speak of 'cortical reflexes' as of an established fact. But they are using the term reflex in a wider sense, in which any and every movement that results from sensory stimulation is denominated a reflex, whether psychical intermediaries are brought into play or not. From this point of view, the voluntary action is sometimes defined as a 'cortical reflex.' It is clear that such an expression deprives the word 'reflex' of all special significance. It is also clear that the retention of the term in its stricter meaning is extremely important; for the origin of a class of movements that are at once purely physiological and yet purposive in character is a real and distinct problem. We cannot, of course, enter upon this question of origin at the present time; we can answer it only when we come to examine the various forms of animal movements. We may, however, point out, in view of the following discussion of the functions of the different central regions, that what holds of man in this connexion does not necessarily hold of the animals. We may lay it down as a general proposition that, in man, the centre at which the idea of the reflex gives way to the idea of the psychically conditioned action is the cerebral cortex. But the law is not universally valid; not even valid for all the vertebrates. It is a result of that progressive centralisation in the ascending direction, of which we have spoken in the preceding Chapter, that the mesencephalic areas which, in man, function simply and solely as reflex centres, appear in the lower vertebrates still to be centres for psychically conditioned movements. Indeed, the facts suggest that in the lowest vertebrates, where the cerebrum as a whole is of very minor importance, even the oblongata and the myel may possibly, up to a certain point, mediate movements of this psychical kind. Lower yet, in the invertebrates, they may proceed from any one of the peripheral ganglia; and in the protozoa they evidently have their seat in the general sensorimotor protoplasmic substance of the body. The centralisation of the psychical functions in the brain, that is, goes pari passu with their decen-[p. 253]tralisation in the bodily organs; and this decentralisation corresponds to an extension of the reflex functions. Hence, in the lowest animals, all movements possess the character, not -- as is sometimes maintained in the interest of certain ingrained dogmas -- of reflexes, but rather of psychically conditioned movements.
§ 3. Automatic Excitations
(a) -- Automatic Excitations in Myel and Oblongata
The phenomena of 'automatic function' are in so far parallel to the phenomena of reflex action that they are processes of a purely physiological character, and accordingly have nothing in common with processes which, like voluntary actions, recollections, etc., present themselves to us in direct experience as 'psychically conditioned.' In this purely physiological sense, the automatic functions are therefore nearly allied to the reflexes. But they differ from them in the point that the automatic stimulation processes take their origin in the nerve centres themselves, and are not released by a stimulus conducted to the centre from without. As a general rule, the motor areas that evince reflex phenomena are also susceptible of automatic excitation. The results of these automatic stimulations need not be muscular movements, or inhibitions of particular movements, but may also take the form of sensations. Hence it is not always easy to discriminate them from reflex excitations, or from the direct effects of external stimuli. For all our senses are continually affected by weak stimuli, which have their ground in the structural conditions of the sense organs themselves, and, so far as the sensory centres are concerned, these weak excitations, such e.g. as are aroused by the pressure exerted in the eye upon the retina, in the labyrinth of the ear upon the sensitive membranes, are, of course, the equivalent of stimulation from the outside. If we rule out cases of this kind; it appears that the sole source of automatic excitation is to be looked for in sudden changes in the chemical constitution of the nervous substance, caused for the most part by alteration of the blood.
As regards the myel, the effects of automatic excitation are shown most clearly by the muscles of certain organs of the nutritive system: e.g. the circular muscles of the blood vessels, whose lumen becomes enlarged after transsection of the myel, and the sphincter muscles of bladder and intestine, where similar results have been observed. The tonic excitations of the skeletal muscles appear, on the other hand, to be exclusively reflex in character (cf. p. 93, above), since transsection of the muscle nerves produces [p. 254] no change in muscular tension, apart from the concomitant twitch and its elastic after-effects. Automatic excitations seem, however, to occur, alongside of reflex excitations, in the peripheral organs that are separated from the central organs proper and provided with independent centres, e.g. in the heart and intestinal muscles (cf. p. 248, above).
The automatic excitations that proceed from the oblongata are of especial importance. Here, too, the reflex centres appear, without exception, to be automatic centres as well. The movements that arise in them are consequently continued, after the sensory portion of the reflex path has been interrupted. Here belong the movements of respiration and heart beat, and the innervation of the blood vessels. All of these processes are connected with two centres, distinct not only in function but also in locality: the respiratory movements with centres of inspiration and expiration, the cardiac movements with centres for acceleration and inhibition of the heart beat, the vascular innervation with centres for constriction and dilatation of the blood vessels. Under such circumstances it seems to be the rule that the one centre acts reflexly while the other combines automatic with reflex functions, or even gives the preference to automatic stimuli: so the inspiratory centre in the case of respiratory movements, the centre for inhibition of heart beat in that of cardiac movements, and the centre for vaso-constriction in that of vascular innervation. It may be that the position of these nerve nidi, and the way in which their blood supply is distributed, render them especially liable to automatic excitations. The normal physiological stimulus to the production of such excitations is, in all probability, that state of the blood which is induced by arrest of breathing or, indeed, by any circumstance that prevents the elimination of the oxidised constituents of the tissues. The presence in the dyspnoeic blood of oxidation products in general, whether of the final product of combustion, carbonic acid, or of lower stages of oxidation as yet unnamed, appears accordingly to constitute it a source of nervous stimulation. The accumulation of these materials excites the inspiratory centre: an inspiration is made, which causes the lungs to distend and thus, in its turn, serves reflexly to excite the centre of expiration (p. 245). This automatic stimulation completes the circle of self-regulating functions, whereby the process of respiration is kept in perpetual activity. The first impulse is given by the change in the constitution of the blood: this acts as an internal stimulus to excite inspiration. The beginning once made, the further periodic course of the whole process continues of its own accord. The expiratory reflex excited by distention of the lung is followed, as the organ collapses, by the inspiratory reflex and at the same time, in consequence of the renewed accumulation [p. 255] of products of oxidation, by renewed automatic stimulation of the inspiratory centre.
We may suppose that the same changes in the composition of the blood condition the automatic innervation of the inhibitory centre for the heart and of the presser centre for the blood vessels. It is ordinarily assumed that the excitations in these two cases are not, as they are in the case of respiration; subject to a rhythmical rise and fall, in consequence of the self-regulation of the process of stimulation, but hold throughout to a uniform level of intensity. This is inferred from the facts that severance of the inhibitory nerves of the heart, the vagus trunks, produces a persistent acceleration of the heart beat, and that severance of the vascular nerves effects a permanent dilatation of the small arteries. But these facts are not incompatible with the theory that the automatic excitation in both cases oscillates between certain upper and lower limits. There are, in reality, numerous phenomena that tell in favour of such a theory: e.g., the alternate constrictions and dilatations that may be observed in the arteries, and that usually disappear after transsection of the nerves; or the connexion between rapidity of pulse and respiration, a connexion which, as we have seen, depends in part upon the changes of volume in the lung and is therefore explicable in reflex terms, but in part also suggests a different origin, seeing that a long-continued arrest of breathing, whether it occur in the position of inspiration or in that of expiration, arrests the heart as well. Moreover, in death by suffocation we always find, besides intensive excitation of the inspiratory muscles, constriction of the blood-vessels and inhibition of the heart beat. We may accordingly conjecture that the automatic excitation of all these oblongata centres depends upon analogous changes in the constitution of the blood. The observed differences may very well have their ground in the relations of the peripheral nerve terminations; for the inspiratory centre stands in connexion with ordinary motor nerves, whereas heart and blood vessels are characterised by the independence of their peripheral innervations. The heart continues to pulsate, though with change of rhythm when separated from all nerves whatsoever; and the vascular wall remains capable, under the same conditions, of alternate constrictions and dilatations. The causes which determine these peripheral excitations are, in all probability, similar to those which regulate the respiratory innervation in the myel and, like the latter, are compounded of automatic and reflex processes while the rhythmical function of the heart and the equilibrium between excitation and inhibition in the vessels are also maintained by some self-regulative mechanism. That is to say, the innervations of lungs, heart and blood vessels are, probably, in so far related to one another that the automatic excitations from which they spring may be referred to one and the same source of origin. The centres for these [p. 256] movements appear to offer especially favourable conditions for the action of the internal stimuli; for no other central area reacts so sensitively to fluctuations in the composition of the blood. In other quarters of the central nervous system, we may suppose, the influence of the blood becomes effective only if the blood supply has been modified from these centres of respiratory, cardiac and vascular innervation, and the changes thus set up form a source of central stimulation. Thus, excitations of the vascular centre, which inhibit the circulation of blood in the brain, are probably, in many instances, the cause of general muscular convulsions. Under such circumstances, the external symptoms are, for the most part, initiated in the pons; sometimes, perhaps, in a more anteriorly situated motor brain-region. The dyspnoeic blood may, however, occasion muscular convulsions of the same kind, though less widely diffused, by stimulation of the myel.
(b) -- Automatic Excitations in the Brain Cortex
Of the parts lying beyond the pons, the centrosensory and centromotor regions of the brain cortex seem to be the principal centres from which, under the appropriate conditions, automatic excitations may proceed. In their case, however, we are never in presence of purely automatic processes, in the physiological sense defined above. The relations of the cerebral cortex to the psychical functions are such that the automatic excitations are connected, in every instance, with conscious processes, -- processes that may: in general, be subsumed under the rubric of psychical association, and that refer us to psychophysical conditions of a very complicated kind. Nevertheless, the part played by automatic stimulation is far from unimportant. It serves to modify the excitability of the cerebral cortex; and the state of cortical excitability largely determines the appearance and course of these psychophysical processes. Among its results, we must mention, in the first place, those phenomena of stimulation that may almost be termed the normal accompaniments of sleep. They show themselves usually, and oftentimes exclusively, as sensory excitations. So arises the customary, sensory form of the dream, in which automatic enhancement of excitability in the sensory centres produces -- always, probably, under the influence of external sense stimuli -- ideas of hallucinatory character. Sometimes motor excitations are also involved: muscular movements occur, ordinarily in the mechanisms of speech, more rarely in the locomotor apparatus, and combine with the phenomena of sensory excitation to form a more or less coherent series of ideas and actions. In all these phenomena, sensory and motor alike, the automatic change of excitability is simply [p. 257] the foundation, upon which the complex psychophysical conditions of the dream consciousness and its outward manifestations are built up. The point of departure of these central changes, which follow the oncoming of sleep, is again to be sought, most probably, in the innervation centres of the oblongata. MOSSO has shown, by observation of cases in which a portion of the skull had been removed, that at the moment of falling asleep the flow of blood to the brain is reduced; and, further, that the supply may, in most instances, be temporarily increased by the application of external sense stimuli, even if these are too weak to arouse the sleeper. The general reduction of the blood flow is, in all probability, the cause of the marked diminution in the excitability of the brain centres, and of the corresponding obscuration of consciousness, that characterise the approach of sleep. Very soon, however, this inhibition of the central functions spreads still further, involving to a certain extent the centres of respiration and heart beat; so that the phenomena of dyspnoea not infrequently make their appearance during sleep. The enhanced excitability of particular central elements of the brain cortex, vouched for by the phantasms of dreaming, may accordingly be ascribed to the direct excitatory influence upon the cortex of the dyspnoeic modification of the blood. It is also possible, in view of the reciprocal relations sustained by the various central areas, that stimulations accidentally set up in a given region of the cortex will produce the more intensive result, the greater the degree of latent excitation in the adjacent parts.
Similar excitations of the cerebral cortex may occur in the waking state; but they are then invariably the result of pathological changes. Here, again, investigation frequently refers us to an abnormal state of the circulation as their ultimate condition. The abnormality may be of local origin, proceeding from the vessels of the meninges or of the brain itself. Local lesions, in particular, set up in the neighbourhood of the sensory centres, are ordinarily attended by corresponding hallucinations. These, however, may also be due to general disturbances of circulation, which appear sometimes as the consequence, sometimes as the cause of psychical derangement; for changes in the innervation of heart and vessels are frequently observed in cases of mental disease. Now all the chronic forms of insanity are connected with more or less serious modifications of [p. 258] the brain cortex; and diffuse affections of the vascular membrane with which the cortex is invested are the most frequent causes of acute psychical disorder. But the phenomena of stimulation accompanying such disorder closely resemble those that normally appear in sleep. They belong, as the latter also belong, partly to the sensory, partly to the motor sphere. The sensory excitation manifests itself in sensations and ideas of the different senses, oftentimes equal in intensity to those that can be caused by external impressions, and therefore indistinguishable from them. These hallucinations are accompanied by changes in the subjective sensations, muscular and organic, upon which the affective disposition largely depends. Motor stimulations show themselves in the form of imperative actions, which are likely to impress the observer by their unwonted energy. Here too, however, as in dreams and dream movements, the enhancement of excitability due to automatic stimulation is combined with further psychophysical processes, which are responsible for the specific contents of the phenomena.
§ 4. Functions of the Mesencephalon and Diencephalon
(a) -- Functions of the Mesencephalon and Diencephalon in the Lower Vertebrates
It is evident from mere inspection, and without recourse to histological methods, that the mesencephalic and diencephalic region, which in man and the higher mammals, more especially in the nearly related primates, cannot compare with the mass of the overarching cerebral hemispheres, forms in the lower vertebrates the most highly developed part of the central organ. Even in the birds and the lower mammals, where the prosencephalon has already attained a considerable size, its relative development is still greater than that of the superior parts (cf. Fig. 54, p. 128). These salient facts of the gross anatomy of the brain are paralleled throughout by functional differences; so that it is far more dangerous in the case of mesencephalon and diencephalon than it is in that of myel and oblongata to argue from symptoms observed in the lower animals to the organisation of the higher, and in particular of man. Yet another difficulty in the way of a functional analysis of this region, whether in the animals or in man, lies in the circumstance that experimental interference and pathological disturbance rarely affect a definite and definitely circumscribed area, but are apt to spread to adjacent parts, -- experimental interference, more especially, involving the crural and coronal fibres that pass upward below and between the thalami and quadrigemina. Hence most of the results of the earlier [p. 259] experiments upon the transsection of these centres leave us uncertain whether the motor derangements observed were really the consequence of the destruction of the parts themselves, or not rather of the interruption of the neighbouring conduction paths. Indeed, the whole method was at fault. The symptoms of stimulation and abrogation do good service in the investigation of conduction paths, and especially of their beginnings in the myel and of their terminations in the cerebral cortex. But in the present case, where the separation of the parts under examination from their surroundings presents extreme difficulty, they can hardly be employed with any prospect of success. As the stimulus method is here, for obvious reasons, practically out of the question, physiology has accordingly come more and more to substitute for the direct an indirect form of the method of abrogation. Instead of asking what functions remain intact after removal of the mesencephalic and diencephalic centres which he has under investigation, the modern physiologist inverts the question, and asks what functions are still left when all the prosencephalic parts that lie above and beyond them have been cut away. He then makes a series of similar observations upon animals of the same species in which the entire central organ has been removed with the exception of oblongata and myel; and, by recording the difference of result in the two cases, is able to reach a conclusion with regard to the functional significance of the intermediate central region. This method was employed long since by FLOURENS upon birds, -- though employed, at first, rather with a view to the determination of the importance of the prosencephalon itself, whose extirpation it involved. It was then applied, systematically, by GOLTZ, in work upon the frog; and has been used by CHRISTIANI  and, still more recently by GOLTZ  again for mammals, and finally by J. STEINER  for vertebrates of all classes. It evidently guarantees a somewhat more reliable result, if not for each individual centre included in the mesencephalic and diencephalic region, at least for this region as a whole.
The observations taken on the lines here laid down prove that the functional importance of the mesencephalic and diencephalic centres through-[p. 260] out the vertebrate series keeps practically even pace with the development of the parts as revealed by gross anatomy. This development is not uniform for the two regions: in the lower orders, the mesencephalon (bigemina or optic lobes) has the preponderance, and the diencephalon (thalamus) is relatively insignificant. Thus, in the entire class of the fishes, with the exception of Amphioxus lanceolatus which stops short at the myel, the mesencephalon appears as the dominant central organ. So long as it remains uninjured, the essential psychical functions are hardly modified. In particular, the animals react quite normally to optical and tactual impressions, and move spontaneously and appropriately. Smell alone is abrogated: the olfactory nerves are, naturally, removed with extirpation of the prosencephalon: and the inception of nourishment, in so far as it is governed by impressions of smell, may in consequence be more or less seriously deranged. Passing to the amphibia, we find at once a marked difference of behaviour in animals whose cerebrum has been removed. One function is, unquestionably, retained by them, and must therefore depend for its effectiveness upon the integrity of the mesencephalon: the function of progression, and the regulation of co-ordinated movements of the whole body. The decerebrised frog sits upright, like the uninjured animal; if made to change its place by the action of cutaneous stimuli, it avoids obstacles laid in its path; and so on. It presents but a single abnormality: that, at first, it neither moves nor takes food of its own accord. At the same time, its behaviour shows two noteworthy features. On the one hand, the functional separation of mesencephalon and diencephalon is becoming clearer; on the other hand, we observe the influence of practice upon the formation of new habits. If the diencephalon is intact, the frog, as SCHRADER remarked, slowly recovers: it begins to catch flies again of its own accord, and continues to improve until at last it is altogether indistinguishable from a normal animal. A bird deprived of the prosencephalon behaves in very much the same way. It, too, as FLOURENS observed many years ago, at first remains motionless: it stands upright, breathes regularly, swallows if it is fed, and reacts to stimuli by co-ordinated movements of flight; but it makes no movements of its own initiative. Here again, however, there is a gradual change of behaviour, if the animal is kept alive for any length of time: it makes restless movements from side to side, avoids obstacles as it moves, and so forth. CHRISTIANI, who was the first to make observations on mammals, found that the rabbit, after removal of all the parts of the brain anterior to the mesencephalon and diencephalon, is similarly capable of reacting appropriately to light stimuli, of avoiding [p. 261] obstacles when stimulated to movements of escape, and of occasionally executing what appear to be spontaneous movements. Finally, a still more thorough-going restoration of function was seen by GOLTZ, in the case of dogs that he had kept alive for a considerable period of time after complete extirpation of the cerebrum. As usual, the animals were entirely passive in the interval immediately following the operation: only the vegetative functions (heart beat, breathing, movements of swallowing upon the introduction of food into the gullet) went on without disturbance from the beginning. The progress of time brought with it, however, a much more complete recovery of active function; and at last the animals moved about in an almost normal manner, reacted to tactual stimuli by barking, got on their feet again if they had fallen down, alternated between sleep and waking, and could be aroused from sleep by sound stimuli. Smell had, it is true, been entirely abrogated with the extirpation of the olfactorius; nevertheless, the dogs fed of their own accord when food was held against their muzzles. Bad-tasting morsels they spat out again. On the other hand, there was never any expression of pleasurable feeling, of attachment, and never any act that could be interpreted as a sign of personal recognition. These were permanently lost.
From these results we must conclude that the mesencephalic and diencephalic region plays a very considerable part in the whole vertebrate series up to the carnivores. It contains a group of important central stations for the colligation of sense impressions with their appropriate movements, -- stations which, like the reflex mechanisms of the myel, continue to function after their severance from the higher central parts. But more than this: its integrity is the condition of the integrity of the simpler psychical functions. The mental loss that the animal suffers by operation is twofold. It loses, on the one hand, the functions connected with determinate sensory nerves that are involved in the lesion caused by removal of the prosencephalon: so the reactions to smell impressions. It loses, on the other hand, the functions which presuppose a manifold connexion of present impressions with past experiences: so the recognition of persons, the feelings of attraction and repulsion, of joy, etc. Some authors, it is true, disregarding the results obtained from decerebrised dogs, and relying on observations made upon anencephalic monsters, localised the feelings and emotions, in man as well as in animals, in the mesencephalic and diencephalic region. But they are guilty of an obvious error in reasoning. They ascribe the response to gustatory stimuli -- mimetic reflexes, which in these pathological cases are left intact -- to concomitant feelings. It is, of course, no more allowable to argue in this way than it would be to interpret any other reflex movement,[p. 262] on account of its apparent purposiveness, as of necessity a conscious and voluntary action.
We see, then, that these middle brain regions are, for the animals in general, something more than centres of complicated reflexes. In the light of the phenomena described above, we must consider them also as centres for the simpler psychically conditioned functions. A more detailed comparison shows, now, that as regards the time of their appearance these phenomena present very striking differences. In the lowest vertebrates, the fishes, removal of the prosencephalon produces no marked change of any kind in the psychical behaviour of the animals. At a somewhat higher level, in amphibia, reptiles and birds, there is, at first, an interruption of the psychical functions; and those that remain intact, since there is no trace of any lasting after-effect and but very slight indication of adaptation to new conditions, might if needs were be interpreted as complicated reflexes. It is, however, evident that in course of time the animal makes a fairly complete functional recovery. The same picture, only with its lines more strongly drawn, may stand, finally, for the mammal. The lapse of psychical functions after the operation is here more pronounced; a longer period is required for recovery; the permanent mental defect is more clearly observable. Nevertheless, the injury is compensated within wide limits. If we are rightly to interpret these phenomena, we must of course remember that all the functions which are permanently lost in the higher mammals -- recognition, expressions of pleasure and attachment -- do not exist at all in the, lower animals. Now this gradual return of the expressions of mental life, in animals endowed with a fairly well developed prosencephalon, admits, if looked at simply by itself, of two explanations. It may be that the operation gives rise to some sort of inhibitory effects, perhaps conditioned upon the injury to the parts, which must be gradually overcome; or it may be that the uninjured remnant of the brain gradually takes on a share of the functions discharged in the normal organism by the prosencephalon. According as we incline to the one or the other of these interpretations, will our estimate of the functional significance of the mesencenphalic and diencephalic regions vary. If we accept the former, these parts will be responsible, throughout the vertebrate series up to the carnivores, for a very considerable proportion of the functions of the brain at large; the animal's recovery will mean, for them, simply a restoration of their original rights. If we accept the latter, their functional activity after recovery will be abnormally increased, because partly vicarious. Now it cannot be denied that there are many facts which tell in favour of the effect of operation, as at least a joint factor in the general result. Radical interferences by operation, and especially interferences with the central organs, are known to affect the functions for a certain period of time. Still, it is [p. 263] hardly probable that the effect of operation is the determining factor in the case before us. The contrast between the proximate effects of the loss and the subsequent state of the animal is too striking and too uniform. Besides, there would be nothing to explain the graduation of phenomena in the animal kingdom: the fact, e.g., that in the frog -- to say nothing of the mammals -- a considerable period of time elapses before complete compensation is observable, whereas in the fishes recovery sets in at once. And lastly, there are numerous phenomena, drawn from all kinds of sources, which prone that injury or loss to the central parts, whether in man or in the animals, may within very wide limits be offset by the vicarious functioning of uninjured organs. We shall see presently that this law of functional substitution is indispensable, if we are to explain the reciprocal relations of the various cerebral areas. It is, then, only natural, in the absence of evidence to the contrary, that we should posit its validity in the present instance for the interrelations of the various parts of the brain. We may add that the general possibility of such vicarious function is inherent in the nature of the quadrigemina and thalami, as intermediate stations upon the direct lines of conduction between the peripheral organs and the cerebral cortex, -- stations in which all the sensory paths, and a large proportion of the motor, are interrupted by the interrelation of neurone chains. Putting all these facts together, we arrive at the following genetic conclusions, as a general point of view from which the various phenomena observable in the vertebrate series may be classified and explained. At the lowest stages of brain development, the mesencephalic and diencephalic region -- especially the former, since the diencephalon is as yet comparatively insignificant -- appears as the principal central organ. Subordinate to this, on the one side, are oblongata and myel. Adjoining it, on the other, as an appendicular structure, is the prosencephalon; originally, we must suppose, an outcome of the separate development of the olfactorius. As we proceed upwards through the vertebrate series, further representations of the conduction paths brought together in the bigemina gradually make their appearance, as superior centres, in the prosencephalon. In proportion as the latter advances, the mesencephalon and the diencephalon -- the latter conditioned in its own development upon the formation of the prosencephalon -- take on the function of intermediate centres, where excitations from the periphery touch off complicated reflexes, and excitations from the prosencephalon evoke reactions depending upon a more extended colligation of impressions. Nevertheless, the possibility remains, up to the higher stages of development, that on the removal of the superior regulatory mechanisms the lower centres may gradually recover some measure of the autonomy of which at a lower level they were in complete possession. Hence it may well happen that in the normal interplay of the central organs, -- so long, that is, as they stand [p. 264] under the dominance of the prosencephalic region, -- these parts of the brain may have no other function than that of complex reflex centres; but that in the absence of the higher regulatory organs they may once more assume the character of independent centres, whose co-operation involves the appearance of psychical functions.
We conclude, therefore, that this part of the brain possesses, under all circumstances, the importance of a centre whose office it is to bring into connexion the principal organs of sense and of movement. Such a view of its functions is, upon the whole, confirmed by the symptoms which ordinarily follow upon its direct removal or impairment. The most striking of these, upon the sensory side, is the blindness which, in mammals, in accordance with the course of the opticus paths, is correlated in particular with the pregemina, including the pregeniculum. Disturbances of movement appear, on the other hand, provided that the lesion does not extend beyond the quadrigemina, to be confined, at least in the mammals, to the muscles of the eyes; the general muscular system of the body is unaffected. If, however, the diencephalon is injured, the general motor derangement is very pronounced. It consists, where the injury is unilateral, in peculiar imperative movements, in which the animal, instead of going straight forwards, turns round in a circle. These circus movements (Reitbahnbewegung, mouvement de manège) are also observed after injury to other parts of the brain, more especially the crura and the cerebellar hemispheres, and after unilateral extirpation of the semicircular canals of the internal ear. In the lower vertebrates, e.g. the frog, the circus movements are invariably made towards the uninjured side. In the invertebrates, too, the principal ganglia behave, in this matter of motor disturbance and its direction, in precisely the same way as the mesencephalon of the lower vertebrates. In the mammals, on the other hand, the rule is that movement is directed towards the injured side, if the anterior portion of the thalamus has been divided, towards the uninjured side, if the section has been made in its posterior portion. Abnormalities have also been observed in the tonicity of the muscles of the body, so that the animal when at rest is not extended, but bends upon itself, the direction of curvature corresponding to that of the circus movements. These movements may take on various forms, according to the special conditions of the injury: they may appear as rolling movements about the longitudinal axis of the body, as 'clock hand' movements, or finally as circus movements proper. They are, we may suppose, occasioned in all cases by an asymmetrical innervation, which however may itself be due to a number of causes: to unilateral increase [p. 265] or decrease of motor innervation, or to the asymmetrical release of reflex movements connected with disturbances of sensitivity. Which of these conditions, or what combination of them, is actually at work cannot, at present, be certainly determined.
The operation never fails to set up this motor derangement. In the higher mammals, we find, further, symptoms of abrogation or diminution of cutaneous sensitivity upon the uninjured side of the body. Such symptoms are always ambiguous, and the results are correspondingly doubtful. On the whole, however, if we look at the phenomena in their entirety, and take into consideration at the same time the defects observed shortly after removal of the prosencephalon and the known facts regarding the course of the conduction paths, we may conclude that the mesencephalic and diencephalic region constitutes, in all the higher vertebrates, an important intermediate station on the road front the deeper lying centres to the prosencephalon; a station for the release, on the one hand, of compound reflexes, more especially of reflexes to visual and auditory stimuli, in which the prosencephalon is not concerned; and, on the other, of centrifugal excitations from the cerebrum, whose components are here co-ordinated in such a way as best to subserve the needs of the organism. In view of the anatomical relations and of the results of experiments with partial extirpation, it is probable that the postgemina represent, in the main, intermediate stations for the acoustic area; the pregemina and pregenicula similar stations for the sense of sight; and the thalami proper stations for the extensive area of the sense of touch. We thus have, in this whole region, a group of nodal points for the function of all the sense departments (with the exception of smell) and of the movements correlated with them. It follows that the mesencephalon and diencephalon, in proportion to their degree of development as compared with that of the prosencephalon, are able between them, after the lapse of the prosencephalic functions, to undertake in their own right the unitary regulation of the processes of the animal life; although certain functions, conditioned exclusively upon the cerebrum, are of course permanently lost. This physiological status implies a concomitant development of psychical processes: the persistence of impressions of sense for a certain time after the cessation of stimulus, the formation of complex perceptions, mediated by associative processes, and the conduct of movements in accordance with impressions received in the more remote past. In other words, centres that originally subserved reflex action and the transmission of impulses have, under pressure of novel conditions, become transformed into independent centres of direction. They are still centres [p. 266] of the second order; but their lesser functional value, as compared with the higher centres whose substitutes they are, depends essentially upon the degree of development attained by these higher centres themselves.
For a long time, the physiology of the mesencephalic and diencephalic region suffered from a misconception. It was insistently held that the functions of these parts were not only analogous, but in the main actually equivalent throughout the vertebrate kingdom; so that, in particular, what held of the animals must also hold of man. The older method, of experiments with direct abrogation, was not competent to remove this error. The necessary change of view has been brought about, gradually, by extirpation experiments on the prosencephalon itself; experiments which, as we remarked above, have really attained in this way a different purpose from that upon which they were originally directed. Physiologists, from FLOURENS to GOLTZ, made these experiments with the primary intention of deriving from the resulting symptoms of abrogation a more exact knowledge of the function of the prosencephalon. But it became more and more evident -- especially, as it happened, in the course of investigations pursued by GOLTZ and his pupils -- that this direct end could be accomplished but very imperfectly, if at all, on account of the direct and indirect consequences that follow the operations, and that make the comparison of the injured with the normal animal anything rather than a problem in simple subtraction. At the same time, it became evident that all such experiments yield most important information regarding the functions of which the uninjured brain remnant is capable. Extirpation experiments are, therefore, still valued by the modern investigator, but they are valued for a different reason. They are not expected to reveal anything of moment concerning the functions of the parts destroyed, but rather to illustrate the possible functions of those that are left intact. It need, however, hardly be said, after the discussion in the text, that these functions are not to be identified forthright with those discharged by the same parts in the normal interplay of the organs. In this connexion, the differences that we find throughout the animal kingdom, the very differences that were formerly overlooked, are of great significance. First and foremost, the differences between the various vertebrate classes, but secondarily and within certain limits -- despite the radically divergent position of the central organs, from the genetic point of view -- the differences among the invertebrates as well, have in many instances shed light upon the far more complicated conditions obtaining in the brains of the higher mammals. The impulse to such comparison came in the first instance from morphology. On the side of physiology, it is the especial merit of J. STEINER to have shown, by his experiments on fishes and on the frog, supplemented by later work on reptiles and invertebrates, how extremely variable is the role assigned to the mesencephalon in the vertebrate series. Setting out from the spinal functions of Amphioxus, which, as we know, has no other central organ than the myel, STEINER has, further, attempted a general theory of the mesencephalic functions at large. But here, unfortunately, his foundation is uncertain, and the structure erected upon it still less secure. In the annelids, he says, the individual metameres and the corresponding terms in the series of ventral ganglia are all equivalent, so that any portion of the worm is, in its own right, just as capable of movement and, apparently, of sensation, as is the whole animal. Amphioxus is, now, to be regarded in the same way: its myel consists simply of a series of [p. 267] equivalent terms, not subordinated to any higher centre. Then, at the next stage, represented by the primitive fish, the shark, and at stages of progressive advance, represented by the other fishes, the myel is brought under the central superintendence of the mesencephalon, which henceforth remains, throughout the vertebrate series, the real directing centre: the prosencephalon is to be considered as merely supplementary. It is true that, in the mammals, the prosencephalon attains a marked preponderance; nevertheless, the mesencephalon and diencephalon contain the centres for the regulation of the whole system of bodily movements, and therefore still hold the part of the central organ proper. In this sense, STEINER defines a 'brain' as "the universal centre of movement, in connexion with the functions of at least one of the higher sensory nerves." The criterion of an 'universal centre of movement' consists, for him, in the occurrence of unilateral forced movements after injury to the one side of the organ. If, then, there is no part of the central organs at which these circus movements can be released, there cannot either be any unitary centre of direction within the nervous system; the entire central organ must consist of a number of equivalent metameres. Now it is plain that the point of departure for all these theoretical considerations is furnished by the annelids. The bodily segments of these animals appear, when divided off, to represent independent vital units, every whit as capable of continued spontaneous movement as was the original, uninjured worm. The annelids, moreover, do not execute circus movements after removal of the dorsal ganglion of the one side. But the only inference that can be drawn from the latter fact is, surely, this: that the occurrence of forced movements is, in all probability, not an universal criterion of the presence of a directing centre from which lower centres are controlled. And further: we have no right to assume the complete functional independence and equivalence of all the terms in the series of ventral ganglia, unless the individual segments move just as independently while they are still connected with the total annelid body as they do after their separation. This, however, is not the case; in the uninjured animal all the metameres move in exact co-ordination with one another. We must, therefore, conclude that the whole chain of ganglia normally functions as an unitary system, of which, if we may judge from the anatomical relations, the dorsal ganglion is the directive centre. In just the same way the myel in Amphioxus is apparently controlled by its most anterior portion as in some sort the equivalent of the brain of the craniota (see p. 252 above). The word 'brain' is, in the first instance, an expression taken over by science from popular parlance, and on that account is, as ordinarily employed, extremely difficult of definition. If we are, nevertheless, to make the attempt, and are not to break with the general application of the term, we must say that the vertebrate brain is not a separate central organ, but rather the complex of all those central organs which share in the direction of the animal functions. In this sense, oblongata, mesencephalon and diencephalon, prosencephalon and cerebellum have equal claims to the name. If, on the other hand, we are to restrict the term 'brain' specifically to the central parts that are able in their own right to maintain the animal life -- though perhaps on a reduced footing -- after the removal of the rest, then the oblongata still has, at any rate, as good a claim as the mesencephalon and diencephalon. The point is that the brain, taken as a whole, is not a centre, but a complex of centres, and of centres so related to one another that, if one of [p. 268] them is lost, a portion of its functions can, as a general rule, be taken over by another.
Experiments on completely decerebrised animals, especially the higher mammals, are of extreme importance, not only for the functions of the mesencephalic and diencephalic region, but also for the more general question of the functional representation of higher by lower centres. We therefore append here a somewhat detailed account of the phenomena observed by GOLTZ in the decerebrised dog that, of all operated on by him, longest survived the operation. The animal was deprived of its left hemisphere in two experiments, performed on the 27th of June and the 13th of November, 1889; the entire right hemisphere was removed on the 17th of June, 1890. It was killed, with a view to post mortem examination, on the 31st of December, 1891. The general result of the autopsy was confirmatory; the cerebral hemispheres had been completely done away with, in part directly by the operations, in part indirectly by subsequent softening of the tissue. The animal had thus lived for more than eighteen months after the final operation. Immediately afterwards, it had been entirely motionless; but the capacity of spontaneous movement returned as early as the third day. The dog moved to and fro in the room, and was able to avoid obstacles laid in its way, without having first run against them. Placed on a smooth floor, it would slip up, but recover itself at once and of its own accord. If its toes were forced into an unnatural position, it corrected the displacement immediately as it began to walk, and stepped with the sole of the foot in the normal way. It lifted its leg, without falling in, from a hole that had been prepared for the purpose of the experiment. It once sustained an accidental injury to one hind paw, and thereafter, until the wound was healed, held up the injured leg in walking, precisely as a normal dog would do. The sense of touch was blunted; but the animal reacted to tactual stimuli of some intensity, though the localisation of the point stimulated remained, it is true, fairly uncertain. If, e.g., the left hind foot were seized, it would snap to the left, but generally in the air, without reaching the hand that held it. The auditory sensitivity was also greatly reduced; nevertheless, the animal could be aroused from sleep by intensive sound impressions. Gustatory stimuli were sensed. Meat dipped in milk and held before its mouth was seized and chewed up; meat dipped in a solution of quinine was taken, but spat out again, with wry movements of the mouth. The sense of smell was, of course, entirely abrogated: the olfactory nerves had been destroyed in the operations. At first, therefore, the dog took nourishment only when food was placed in its mouth. Later on, it became accustomed to seize and gulp down bits of meat, and to drink milk, as soon as its muzzle was brought in contact with them. It ate and drank of the solid and liquid food thus offered until its appetite was satisfied; it would then lie down and go to sleep. The functions of the sense of sight were shown -- in addition to the avoidance of obstacles, mentioned above -- in the reaction of the pupils to light stimuli. On the other hand, the animal was wholly insensitive to threatening gestures and movements, and to other animals presented for its notice. Consistently with this behaviour, it remained till its last day dull and apathetic. There was no question of any real 'cognition' and 'recognition' of the objects about it. The only expressions of feeling were snarling and biting when intensive stimulation was applied to the skin, and a tendency to restlessness under the influence of hunger. Nevertheless, the avoidance of [p. 269] obstacles shows an adaptation of movement to the varying conditions of sense impressions; and the same fact is brought out still more clearly in the following experiment. Two long boards were put together to form a blind passage-way, about twice as long as the animal itself, and so narrow that it could not turn round. When the dog was introduced into this passage-way, it first walked to the farther end, and ran against the wall. For some time, it reared up vainly against this obstacle; but presently it began to back out, and finally, by this crablike movement, reached the open. Of all experiments on decerebrised animals, this is, without doubt, the experiment whose result seems to approach most nearly to what is termed an 'expression of intelligence.' Nevertheless, it is plain that in this case, as in the others, the adaptation of the reacting movements to the sensory stimuli are still confined within limits where it is out of place to speak of any real 'reflection' -- of a choice between different possibilities. The symptoms themselves, considered solely by themselves, might, naturally, be interpreted as voluntary actions. But it is another question whether the whole context in which the phenomena appeared permits of such an interpretation. And this question must, surely, be answered in the negative, for the same reason that we decline, e.g., to ascribe the avoidance of obstacles to a true 'cognition' of the objects, -- the cognition in this instance being disproved by other symptoms. If, however, we rule out expressions of intelligence and voluntary actions, in the strict meanings of those terms, this attitude must not, of course, be construed as a denial that the actions of the decerebrised animal are, in part, conscious processes. On the contrary, it must be regarded as, at the least, extremely probable that they may be interpreted as conscious and, in this sense, not merely as purely mechanical reflexes. We cannot, however, enter upon this question with any fulness until we come to our psychological discussion of the idea of 'consciousness' (cf. Part V., Ch. xviii., below).
(b) -- Functions of the Mesencephalon and Diencephalon in Man
In man, and indeed in all the other primates, who in this regard stand upon practically the same level as man, the preponderance of the prosencephalon, which becomes the more marked the higher we ascend in the vertebrate series, has reached a limit where the centres of mid brain and 'tween brain retain least of their original relative independence. This statement is justified both by the relations of the conduction paths and by the nature of the disturbances produced by pathological defects. We cannot, it is true, -- and the reasons are obvious, -- expect to find human cases that shall reproduce the conditions of total extirpation of the prosencephalon, with permanent retention of function in the middle brain regions. But in cases of restricted lesion of the quadrigemina and thalami, it would seem that a restitution of functions by way of vicarious representation in co-ordinated or superior parts may occur very extensively in the human brain. At the same time, the close connexion of the pregemina with the visual functions is evidenced by the derangement of ocular movements that accompanies injury to these parts; while disturbances of visual sensitivity in man appear, for the most part, only when the geniculum is involved. In individual cases -- and the [p. 270] result accords with what we know of the course of the conduction paths -- auditory disturbances have been observed after injury to the postgemina. Lesions of the thalami, as might, again, be expected from the anatomical facts and from the results of experiments on animals, are followed by anæsthesia or by motor disturbances or by both combined. Sometimes, it is true, affections of the thalami run their course without any sign of disturbance whatsoever: a fact that testifies to the wide range of vicarious functioning possible, in this particular instance, within the human brain, and that constitutes a marked quantitative difference between man and the animals, in which the phenomena of abrogation are much more intensive. A second and still more striking difference is this: that the symptoms which in experiments on animals, from the fishes up to the mammals, are set up with the greatest uniformity by unilateral lesions of this area -- the imperative circular movements -- are represented in man, at the best, only by such reduced and vestigial forms as a permanent deflection of the eyes or an unilateral execution of mimetic movements. The determining factors in this result are apparently two: on the one hand, the voluntary suppression of the symptoms, and, on the other, the greater scope of the automatic regulations and functional substitutions that, in the human brain, counteract the disturbances in question. Both factors indicate that, while the basal functions of this region of the human brain correspond to those discharged by the same region throughout the animal series, still its relative importance, as compared with the superior centres, has now become less. Compound reflex centres for the principal sense departments, sight, hearing and touch; and comprehensive regulatory centres for the motor excitations issuing from the higher parts of the brain: these the mesencephalon and diencephalon have remained. But other regulatory mechanisms, and the independent processes of release within the prosencephalon, have increased in importance alongside of them: so that their assumption in man of such psychical function as has been observed to persist in the dog alter removal of the prosencephalic parts can hardly be regarded as probable.
(c) -- Striatum and Lenticula
Striatum and lenticula belong, morphologically, to the prosencephalon (pp. 128 f.). But little is known of their function. They appear, however, to be cortical areas, sunk into the substance of the hemisphere, and specifically correlated with the mesencephalic and diencephalic ganglia. This view is suggested by the extent of their fibre connexions, more especially with the thalami (Fig. 74, p. 179). It is borne out, further, by the phenomena observed in experiments on animals, and in cases of lesion in [p. 271] man, in which these structures are involved. The phenomena consist always of paralytic symptoms or, when excitatory influences are at work, of exaggeration of movement. Here again, however, and particularly in the case of man, the phenomena of abrogation are most pronounced when the lesion has been rapidly produced: slow growing tumours may, under certain circumstances, run their course without giving rise to any symptom whatever. NOTHNAGEL found, further, that mechanical or chemical stimulation of the striatum of the rabbit occasioned hurried running movements. MAGENDIE observed the same result after complete removal of the striatum. Anæsthesia, on the contrary, does not appear to be a consequence of injury to these structures. For the rest, the intensive disturbances that ordinarily follow upon sudden lesions of the striatum are not beyond suspicion; they may be due to implication of the pyramidal paths ascending in the capsula to the cerebral cortex. Besides these relations to the mid brain and 'tween brain, the anatomical facts indicate a further connexion with the cerebellum. As a matter of fact, atrophy of the striatum, and especially of the lenticula, has been observed in cases of congenital failure of the cerebellum.
§ 5. Functions of the Cerebellum
The functions of the cerebellum form one of the most obscure chapters in the physiology of the central organs. The obscurity is intelligible, when we remember the extensive connexions of the cerebellum with numerous other central arts, -- with the oblongata, with the mesencephalon and diencephalon, and above all with the cerebral cortex. For, on the one hand, these connexions make it difficult to determine whether destruction of the other brain centres involves abrogation of the corresponding cerebellar functions. And, on the other, we are left equally in doubt whether the disturbances observed in cases of lesion or defect of the cerebellum are not due, in part at least, to the indirect implication of other parts of the brain with which it stands in connexion. To these is added the further difficulty, that the cerebellar derangements appear to be peculiarly easy of compensation by the enhancement or substitution of function in other central parts. We have, therefore, as many reasons to overestimate as we have to underestimate the importance of this organ; and our uncertainty is not a little increased by the ambiguity of symptoms, which characterises all the phenomena of central abrogation, but is especially marked in this particular case.[p. 272]
These symptoms themselves consist, for the most part, in motor disturbances. Complete extirpation of the cerebellum in animals renders all movements vacillating and uncertain, staggering or tremulous, though the influence of the will upon the individual muscle groups is not destroyed. Transsection of various parts of the cerebellum, as well as of the cerebellar peduncles, -- whose radiations are, for that matter, involved in all deep-going injuries to the organ, -- is ordinarily followed by unilateral motor derangement. If the section passes through the most anterior portion of the vermis, the animals fall forwards; in spontaneous movements, the body is bent over anteriorly, always ready to fall and fall again. If it passes through the posterior portion of the vermis, the body is bent backwards, and there is a tendency to backward movements. If the one pileum is injured or removed, the animal falls towards the opposite side, owing to unilateral contraction of the corresponding muscles; violent movements of rotation about the long axis of the body are apt to follow. There occur also, at the moment of operation, convulsive movements of the eyes, usually succeeded by a permanent detection. These abrogation symptoms agree, upon the whole, with the phenomena of stimulation observed with electrical excitation of various parts of the cerebellar cortex. Both alike are, without exception, same-sided, in contradistinction to the consequences of cerebral injury, which appear upon the opposite side of the body. The stimulation phenomena consist in spasmodic movements of the head, the vertebral column, and the eyes.
As regards man, clinical experience is in accord with the results of the observations on animals. Motor disturbances are, again, the most constant symptom. They consist, chiefly, in an uncertain and vacillating gait, sometimes also in similar movements of the head and eyes. The arms appear to be less seriously involved; and it is but seldom that we observe, in man, those violent rotatory movements that, in the animals, accompany unilateral lesions of the pilea or the medipeduncles. For the rest, the motor disturbances in man are most intensive when the vermis is the seat of injury; while affections of the pileum of either side, especially if the change is merely local, may run their course without symptoms of any kind. Serious derangement occurs, seemingly, only with complete functional disability of the pilea, or in the rare cases of atrophy of the entire organ. Under such circumstances, however, the symptoms are not confined to motor dis-[p. 273]turbances; they become exceedingly complicated, and interpretation is correspondingly difficult. Disturbances of cutaneous sensibility do not appear to result from affections that remain limited to the cerebellum, not even from total atrophy of the organ. On the other hand, a characteristic subjective symptom, more frequently connected with disease of the human cerebellum than with other central disorders, is the dizziness that accompanies the motor disturbances. It is therefore probable that the attacks of dizziness induced in the healthy subject by the passage of a strong galvanic current through the occiput are due, in part at least, to its influence upon the cerebellum. And for the same reason we may suspect that this organ is involved in the dizziness produced by certain toxic agencies. Now there are, in general, two conditions under which the phenomena of dizziness may be manifested: first, the functional derangement of certain peripheral sensory apparatus, whose impressions mediate the arousal of sensations that generate the idea of the static equilibrium of the body during rest and motion; and, secondly, such functional disorders of central areas as are in any way calculated to alter the normal relation subsisting between sense impressions and movements or ideas of movement. We shall presently become familiar with a sensory apparatus of the former kind in the ampullæ and canals of the labyrinth of the ear. On the other hand, we appear to have in the cerebellum not the sole, but certainly the most frequent central seat of symptoms of dizziness. When we remember how near together are the labyrinth of the ear and this central organ, we can readily understand that the two forms of disturbance of equilibrium are difficult to discriminate. Besides, we have every reason to believe that they are functionally connected: the vestibular nerve, that supplies the vestibule and canals with sensory fibres, sends a large number of representatives to the cerebellum. These relations to the vestibular division of the labyrinth are, perhaps, our best means of accounting for the influence of the cerebellum [p. 274] upon bodily movements. We know that all the other sense departments, and more especially those that mediate our spatial apprehension of sensory impressions, the senses of sight and touch, find abundant representation in it. And we find that where dizziness is set up by the action of definitely demonstrable subjective or objective causes, these may ordinarily be traced back to one general condition: disturbance of the normal correlation of sense impressions and bodily movements. Again, however, this disturbance may, in the individual case, be brought about, centrally and peripherally, in a great variety of ways. A man may be made dizzy by walking on the ice, if he is not accustomed to it. The uncertainty of vision that goes with amblyopia or strabismus, or that may he induced in a normal-sighted person by covering the one eye, is not infrequently attended by dizziness. The symptoms are still more evident in the walking movements of patients whose tactual sensations are dulled or destroyed by a degeneration of the dorsal columns of the myel. In such cases, the resistance of the ground is not sensed in the accustomed way: the patients lose their equilibrium; they stagger, and try to save themselves from a fall by balancing with the arms. These phenomena show, at the same time, the indispensableness of the co-ordination of sense impression and movement for the correct execution not only of involuntary, but also of voluntary movements. In the latter, too, it is as a rule only the end to be attained that is clearly conscious; the means whereby this end is reached are entrusted to the automatic working of a motor mechanism, where movement interlocks with movement in the right order and to the right purpose. Each separate act in a compound voluntary action reveals, accordingly a precise adaptation to the impressions that we receive from our own body and from external objects. But since the voluntary action is directed exclusively upon the end to he attained, the sense impressions that regulate the movements do not, ordinarily, take any part in the idea of movement. Even the sudden lapse of the regulatory impression is, in most instances, perceived only indirectly, by way of the consequent motor disturbance and the subjective phenomena dependent upon it.
Disturbances of movement due to central causes may now, in general, be brought about in four different ways. They may (1) be paralytic phenomena, i.e. they may be occasioned by a partial abrogation of voluntary movements. They may (2) appear as purely anæsthesic symptoms. They may (3) consist of disturbances of motor co-ordination. Or they may (4) result from disturbance of the normal relation obtaining between sensations and the movements depending upon them. The first of these possibilities is ruled out at once, since paralytic symptoms do not occur after removal [p. 275] of the cerebellum or of separate parts of it; besides, dizziness is never observed in the train of purely motor disabilities. The second seems to promise better. Indeed, it has to a certain extent found acceptance; some authors have conjectured that the cerebellum is an organ of what is termed the 'muscular sense.' But this view can hardly be reconciled with the fact that in cases of atrophy of the cerebellum in man, and after total extirpation of the organ in animals, the capacity of active movements of locomotion is still retained; the movements may be vacillating and uncertain, but they nevertheless allow us to posit a certain degree of sensation in the locomotor muscles. The abrogation of other sensations is equally out of the question. The third interpretation of the cerebellum, as centre of motor co-ordination, was first put forward by FLOURENS, whose views have held their own, down to the most recent times, among physiologists and clinicians. But, first, this definition is too indeterminate to characterise the specific form of co-ordination mediated by the cerebellum. There is no single central motor area, from the myel upwards, that is not the seat of some sort of motor co-ordination. Secondly, the phenomena of dizziness also tell against FLOURENS' interpretation. They indicate that some kind of sensory disturbance is always involved along with the motor. We are thus forced to the conclusion that the fourth of the above hypotheses is the most probable: the hypothesis that inhibition of function in the cerebellum interferes with the action of those sensory impressions that exercise a direct regulatory influence upon the motor innervation proceeding from the cerebrum.
The acceptance of this hypothesis removes various difficulties. Thus, we can explain at once how it comes about that the disturbances produced by lesions of the cerebellum resemble the symptoms due to partial anæsthesia, and yet differ from them on the important point that abrogation of sensations never makes its appearance among the cerebellar phenomena. Where all conscious sensations persist, the only impressions that can be supposed to lapse are those that act upon movement directly and without previous translation into conscious sensations. Voluntary movements as such are as little affected as sensations; even after complete destruction of the cerebellum, the will retains its right of control over each individual muscle. This explains, again, how it is that the disturbances set up by injury to the cerebellum may gradually be compensated. Compensation takes place in this way, that the movements are regulated afresh by the conscious sensations that persist unimpaired. But a certain clumsiness and uncertainty never disappear. It is evident, as one watches, that the [p. 276] movements must always proceed from a sort of reflection. The immediacy and certainty of movement shown by the uninjured animal are either lost or, if they may in some measure be regained, must be acquired slowly and gradually, as the result of a long continued course of renewed practice. Here too, therefore, the principle of the manifold representation of the bodily organs in the brain is seen in opposition. The cerebellum appears to be intended for the direct regulation of voluntary movements by sense impressions. If this hypothesis be correct, it will, accordingly, be the central organ in which the bodily movements incited from the cerebrum are brought into harmony with the position of the animal body in space. This conception agrees sufficiently well with our anatomical knowledge of the course of the lines of conduction, incoming and outgoing. In the postpeduncles the cerebellum receives a representation of the general sensory path, reinforced, in all probability, by fibres from the optic nerve and the most anterior sensory cranial nerves which run in the valvula and the prepeduncles. Its connexion anteriorly is effected by the prepeduncles and medipeduncles, by which it is united partly to the anterior brain ganglia, partly to the most diverse regions of the cerebral cortex. Finally, the extensive representations of the auditory nerve in the cerebellum (Fig. 77, p. 183) may be brought under the same point of view. For if the cerebellum deflects at all that sensory secondary path whose office it is to conduct impressions that influence voluntary movement directly, and not indirectly, by way of conscious sensations, then we shall certainly expect to find that this same path contains a representation of the eighth cranial nerve. The acusticus is precisely the sensory nerve that gives certain objective sense impressions a specific relation to movement; our movements adapt themselves involuntarily, in a corresponding rhythm, to rhythmical impressions of sound.
The question of the functions of the cerebellum cannot be answered, at the present time, with any degree of finality. The one point upon which physiologists are fairly unanimous is that this organ is set off in relative independence, anatomically and functionally, from the other parts of the central organ, and more especially from the cerebrum: so that no single function -- in particular, therefore, neither sensation nor movement -- is wholly abrogated even after its complete elimination, though profound derangements are produced in the co-ordinations of function. But this very fact of relative independence, which in man and the higher animals must be connected with a position of high functional importance, -- a position attested, in any case, by the structure and volume [p. 277] of the organ -- renders the exact determination of the nature of the 'co-ordinations' or 'regulations' effected by the cerebellum a matter of extreme difficulty; and it is not altogether surprising that a good many of the physiologies are still satisfied to stop short at these indefinite terms, -- terms that apply more or less to every central organ, and are therefore tolerably non-committal in the particular case. This position has been attacked, and rightly attacked, by LUCIANI. Aiming from the first at a definiteness of statement that should match the preceding indefiniteness, LUCIANI undertook to analyse the phenomena, so far as possible, along all their various lines, and thus to refer them to distinct groups of symptoms. He has thus been led to distinguish three principal symptoms of abrogation, which he regards as characteristic of cerebellar lesions: asthenia, atony, and astasia. The movements lack their normal energy (asthenia); the tonus of the muscles is lowered (atony); and the movements are uncertain and incoherent (astasia). It has been objected, with some justice, to this characterisation, that the symptoms which it discriminates are, in part at least, closely interconnected: atonia and asthenia, e.g., always occur together. But if the three terms are considered simply as collective expressions for certain partial states, they may be accepted as really denoting the essential features of the cerebellar symptoms. For the interpretation of the phenomena, however, the emphasis must fall, without any question, upon that member of the triad which is at once the most characteristic and also, unfortunately, the most complicated, upon 'astasia.' LUCIANI seems here, in some measure, to have missed the true perspective; he lays most weight upon the first two symptoms, -- which, no doubt, admit of a simpler interpretation -- asthenia and atony. As a result of this mistake, he is inclined to regard the cerebellum as primarily an apparatus for the production of nervous force, an 'auxiliary' or 'intensificatory system' for the whole cerebrospinal organ, which is not the seat of any specific or peculiar functions, but reinforces the functional activity of the entire nervous system. In support of this view, he adduces the trophic disturbances that appear, in course of time, more especially after complete extirpation of the cerebellum, and that ordinarily take the form of muscular atrophy, cutaneous inflammations, decubitus, etc. Now these disturbances, as well as the striking lack of motor energy that perhaps stands in a certain relation to them, are unquestionably very important symptoms. But the possibility still remains that the 'atony' and 'astasia' of movement are interconnected phenomena, in which a part is played by the influence of sensory impressions. We saw, when we were discussing the myel, that the phenomena of tonus are straitly conditioned upon the continued effect of such impressions (p. 93). And trophic disturbances, of the kind observed after extirpation of the cerebellum, appear in all cases of permanent derangement of innervation; they result from the disability of sensory as well as of motor nerves; and they appear always to involve the co-operation of direct trophic influences, exerted by the nerve centres, and of indirect, which have their source in the abrogation of functions. LUCIANI lays special stress upon the fact, established by his observations, that dogs whose cerebellum has been destroyed are still able, when thrown into the water, to make the normal movements of swimming. But this experiment merely confirms, in a very complete way, the fact that all cutaneous impressions can be sensed, and all locomotor movements voluntarily [p. 278] performed, without assistance from the cerebellum. Swimming is precisely the form of movement that may, under certain circumstances, bring into action a continuous voluntary regulation, compensating any inco-ordinations that have arisen involuntarily, for the reason that an intermission of movement means in its case the danger of drowning. The animal that constantly staggers as it attempts to walk or run is, in swimming, compelled at every movement to maintain itself above water by a maximal effort of will.
The view here taken of the cerebellar functions is in all essential points the same as that developed by the author in the first edition of this work. It finds striking confirmation in the statements made by KAHLER and PICK, from the pathological standpoint, concerning the relation of other forms of 'ataxia,' as it is termed, to the cerebellar symptoms. HITZIG, too, in his interpretation of cerebellar dizziness, seems to take up a very similar position. In any attempt at explanation of this symptom, and, indeed, of the abrogation phenomena at large, especial attention must, in the author's opinion, be paid to the two facts brought out just now: that, in the case of voluntary impulses proceeding from the cerebrum, the individual terms in the series of purposive co-ordinations and regulations of the movements always succeed one another, under ordinary conditions, in independence of the will, i.e. automatically; and that they must always, on the other hand, take their direction from the sensory impressions received by the organism.
The impressions conveyed to the central organs may, according to circumstances, be clearly or obscurely conscious, -- may, in many instances, fail to come to consciousness at all. But, at any rate, it is not in consciousness that they are transformed into the motor impulses whose direction they determine. From this point of view we might, perhaps, characterise the cerebellum outright as an auxiliary organ which relieves the cerebrum of a large number of secondary functions: functions that were originally practised under the continuous control of the will, and that in consequence can always be partially resumed by the cerebrum itself. As for the first stage of practice, it may have occurred here, as in many other cases, either in the course of the individual lifetime or in the previous life history of the species, which has left its permanent traces, if anywhere, certainly in the organisation of the central parts. To ascribe to the cerebellum itself any share in conscious functions, or to endow it, as some have done, with a separate consciousness of the second order, a 'subconsciousness' is, as in the light of these arguments it seems to the author, entirely unwarranted. For the fact before us is that the cerebellum has developed into a centre of sensorimotor regulation, and that in the course of this development the individual co-ordinations of the separate acts of movement with the impressions of sense, all purposive and all subordinate to the ultimate end of the voluntary action, have gradually been withdrawn from consciousness. And there is, upon the whole, only one way in which this process can be envisaged we must suppose that, under the influence of definitely directed cerebral innervations, there has developed a central mechanism, automatic in unction, whose office it is to transmit the first, and only the first, discharging impulses to an auxiliary centre; end that this auxiliary centre is endowed with self-regulating apparatus, again [p. 279] automatic in function, which adapt each several movement to the sense impressions coming in at the particular moment. These impressions may, of course, either come to consciousness by the way or remain unconscious: the former, if the conditions favour their special conduction to the sensory centre, the latter, if they are against it, or if the conduction is somewhere inhibited: for the self-regulations as such the matter is indifferent. On the other hand, it may very well happen, as a consequence of the direct conveyance of sensations to the cerebrum and of its response to them, that disturbances in the cerebellar mechanism of the sensorimotor self-regulations are presently compensated. Such compensation will, in particular, always be possible where the lesions are simply partial, so that a new course of practice may be entered upon and novel co-ordinations established. Where, on the contrary, the entire cerebellum is thrown out, a large draft upon the cerebral functions will suffice to hold the disturbances in check and so to mitigate the symptoms: but we can, it is true, expect nothing more.
We suppose, then, that these self-regulations of the voluntary movements are in some way mediated by the cerebellum. If, now, we are asked to give an account of them in detail, me must reply that the question is very difficult to answer, all the more since there is still much obscurity surrounding the directions and terminations of the conduction paths that meet within the organ. The anatomical relations suggest, and we may accept the suggestion as a provisional hypothesis, that the cerebellum, on the one hand, receives centripetal paths, derived from every sensitive portion of the body, and, on the other, sends out intracentral (as regards the organ itself, centrifugal) paths to every centromotor region of the cerebral cortex. We may imagine, accordingly, that the sensory components functioning in a movement, more especially sensations of touch and movement, are in the cerebellum united into a single resultant; and that this is then conducted onwards to the cerebral cortex, and makes connexion with the centromotor processes of discharge which are there in course. Thus, the regular sequence of walking movements is at every stage dependent upon the condition that the sensory impressions produced at each step by the movement itself are repeated in uniform succession. Suppose, now, that such a rhythmical sequence is summated to form a resultant which connects automatically with the voluntary impulses; and suppose that it remains unchanged so long as its components persist without change, while it varies at once when and as its components vary. We should then have, physiologically, a mechanism of self-regulation which at one and the same time reinforces and relieves the centromotor functions of the cerebral centres; and we should be able, psychologically, to explain by appeal to it the automatic, unconscious character of these self-regulations of our movements, which still leaves room for voluntary corrections and novel courses of practice.
Over and above its influence upon the bodily movements, -- of whose reality there can be no doubt, however various may be the interpretations put upon it, -- the cerebellum has at times been accredited with functions of an entirely different order. Thus, the disturbances of intelligence observed in cases where the organ is lacking, combined perhaps with the anatomical fact that in the medipeduncles the cerebellum has extensive connexions with the prosencephalon, has persuaded several authors to attribute to it a share in what are called [p. 280] the 'intellectual' functions. Apart, however, from these isolated observations, which may very probably be explained by concomitant affections of other parts of the brain, the hypothesis has no facts that are at all definite to support it. The view held by GALL and his pupils, that the cerebellum stands in relation to the sexual functions, is hardly held by any physiologist at the present day. The uncritical way in which GALL himself, and still more the phrenologists who followed him, -- COMBE, for instance, -- heaped together quotations from older authors, records of cases that had not been properly investigated, and observations in which the suspicion of self-delusion forces itself irresistibly upon the reader, -- the whole forming a mass of evidential matter that should be impressive solely by its bulk, -- would of itself forbid our deviating any attention to their writings, even if we did not find upon every page the mark of inveterate prepossession. It should be mentioned, on the other side, that, now and again, observers who cannot be accused of any similar prejudice, men like R. WAGNER  and LUSSANA, have regarded as possible this relation of the cerebellum to the sexual functions; though their standpoint, in making this admission, has generally been that the phrenological hypothesis cannot be certainly refuted. But this negative instance does not, of course, furnish any valid argument; and the general uncertainty of our knowledge of the organ necessarily implies that conjectures regarding its functions, of whatever nature they may be, cannot easily be met by apodeictic proof of the contrary. This does not mean, however, that they have become anything more than mere conjectures. Moreover, the argument from the indemonstrability of the opposite can be rebutted, in the present case, by a sufficient number of positive instances, both experimental and pathological. LUCIANI was able entirely to extirpate the cerebellum in dogs without producing a disturbance of the sexual impulse; in many cases he observed an actual enhancement of the sexual phenomena. The statistics of cerebellar tumours in man have, also, failed to yield the slightest confirmation of the phrenologists' view. Finally, the symptomatology of cerebellar affections, so far as it is given objectively, on the ground of observations, affords no hint of sexual reference.
§ 6. Functions of the Cerebral Hemispheres
(a) -- Phenomena of Abrogation after Partial Destruction of the Prosencephalon
Our knowledge of the functions of mesencephalon and diencephalon is, it will be remembered, mainly derived from observations of the psychophysical activities left intact after removal of the prosencephalon (pp. 259 ff.). These same observations may, of course, be turned to account for a [p. 281] theory of the functions of the cerebral hemispheres themselves. Indeed, the results have, as a matter of fact, been applied more often to this than to the former purpose. It is clear, however, that the positive judgment of the persistence of certain activities is more definite and reliable than the negative judgment of their disappearance. Moreover, the prosencephalon is, obviously, far more seriously affected than are the lower lying brain centres by the indirect consequences of operation: whether these are the immediate disturbances produced by the diffusion of excitatory or inhibitory effects, or phenomena of a more gradual growth, changes wrought by compensation and substitution. In view of these facts, the symptoms of abrogation observed with defects of certain parts of the cerebral hemispheres cannot be accepted, without further examination, as the basis of inference regarding the functions of the parts in question. Physiological experiment and pathological observation show, both alike, that local lesions of the cerebrum are not necessarily followed by perceptible alteration of functions. If any extensive portion of the tissue is removed the animals appear heavy and stupid: but this change, too, disappears with time, very rapidly in the lower vertebrates, gradually in the higher, and if some small remnant of the cerebrum has been left uninjured may seemingly, as high up as the carnivores, give place to a complete restoration of functional capacity. A pigeon, from whose brain considerable masses of the cerebral lobes have been removed, is, after the lapse of days or weeks, indistinguishable from a normal animal. In rabbits, and still more in dogs, the mental dullness and general motor inertia are more evident than in birds. In man himself, textural changes of limited extent, if they are of gradual growth, sometimes run their course without external symptom. More extensive injuries, however, are, it is true, always accompanied in man by chronic disturbance of voluntary movement, of sense, or of the psychical functions.
These abrogation phenomena and their compensations are of peculiar interest when the injuries from which they result are of definite character and considerable extent. Large portions of the brain substance may be lost, and the animal, notwithstanding, make a complete functional recovery. Thus GOLTZ found that dogs which he had deprived of the whole of one cerebral hemisphere conducted themselves, some months after the operation, in very much the same way as if they were normal animals. There was a reduction of the cutaneous sensitivity on the opposite side; and when the animal was free to choose between the movements of its extremities, it preferred as a rule to use the muscles of the same side. The per-[p. 281]ceptions of sight and hearing had also become uncertain, though they were by no means destroyed. GOLTZ afterwards made experiments, with like result, upon a monkey (Rhesus), which was almost entirely deprived of one hemisphere. Where a considerable portion of both hemispheres is removed, the symptoms of disturbance are more acute, and at the same time take a definite direction. Thus, dogs deprived of both frontal lobes gave marked indications of motor derangement. The movements were awkward and clumsy, though the capacity of movement was not abrogated. There was no change of sensation. Extirpation of the two occipital lobes, on the other hand, produced disturbances of vision, which appeared, however, to consist less in an abrogation of sensitivity to light than in a serious impairment of the perceptual functions (cf. p. 196 above). In both cases, whether the frontal or the occipital lobes were removed, the intelligence of the animals also seemed to be somewhat diminished, though it was never wholly destroyed. As a general rule, emotive symptoms of like and dislike were still manifested: in this respect, therefore, there is a radical difference between these animals and the dog whose cerebrum was removed entire (cf. pp. 261, 268). At the same time, there were signs of emotive disturbance, which varied characteristically according as the frontal or occipital lobes were removed: in the former case, the animals appeared unusually irritable, -- a fact that may, perhaps, be brought into connexion with the coincident symptoms of hyperæsthesia; in the latter, they became apathetic, probably in consequence of their partial anæsthesia; quarrelsome dogs were rendered, to all appearance, good-tempered, though it is true that they were also uninterested.
The disturbances observed in man as the result of extensive cerebral deficiency appear, on the whole, to resemble these set up by operation in animals. This is true, more especially, of cases in which the one half of the cerebrum is wholly destroyed. Several instances are on record, in the literature of pathology, in which this condition was induced by external injury or by changes due to disease, and the patient nevertheless lived on for some period of time. Under such circumstances, the opposite side of the body was, of course, completely paralysed, owing to the decussation of the conduction paths. The intellectual functions, on the other hand, showed, so the report declares, no noticeable alteration. The only points signalised are incapacity for mental exertion, and an unusually rapid onset of mental fatigue. Unfortunately, however, in no one of these cases, which all belong to the older medical literature, was the patient subjected to any [p. 283] accurate functional examination: so that we can draw from them simply the general conclusion that there occurs in man a partial compensation of the disturbances, similar to that observed in the corresponding experiments on animals. The abrogation symptoms that appear when the anterior or posterior portion of the cerebrum is wanting are also, it would seem, in agreement with the results of experiments on animals, both in psychical regard and with respect to the derangement of motor and sensory capacity. At the same time, the disturbances in this case are of a much more complicated kind, and their psychological analysis is accordingly too defective to allow of any certainty of inference. All the more prominent, on this account, is the position of certain territories of the cerebral cortex, which are connected with definite psychophysical activities of a compound order. These we shall discuss presently with some fullness: they are the only instances in which, in the present state of our knowledge, detailed functional analysis is possible.
All these observations, which refer to the consequence of more or less extensive cerebral lesions, are, it is clear, of but comparatively slight importance for an appreciation of the functions whether of the hemispheres as a whole or of particular regions of the cerebrum. Their chief interest really lies in the evidence which they supply of the existence of very complete arrangements for the compensation of the disturbances. We have seen that, in the lower vertebrates, and even in many of the mammals, such compensation is rendered possible, after removal of the entire prosencephalon, by the vicarious function of diencephalon and mesencephalon, which thus become autonomous centres of greatly increased activity. Within the cerebral hemispheres themselves, however, the possibility of substitution evidently goes much further. Under favourable conditions, more or less complete adjustment may be made, even in the human brain, to quite considerable defects. At the same time, we must accept the corollary that the definitive phenomena of abrogation, in cases of partial lesion of the cerebral hemispheres, may be turned to theoretical account only with the greatest reserve, and that our study of the separate functional departments of the cerebrum must be based rather upon the transitory than upon the chronic disturbances that follow in the train of cortical lesions. This point will appear more clearly when we undertake the special analysis of the functions of vision and speech.
(b) -- Phenomena of Abrogation after Total Loss of the Cerebral Hemispheres
The phenomena of abrogation that result from the loss of certain parts of the cerebrum are, as we have seen, of doubtful significance; the in-[p. 284]fluences of compensation are incalculable. On the other hand, the symptoms that follow upon total loss of the prosencephalon have a definitive value, save only in the case of those animals in which this defect also can be concealed by the vicarious functioning of mesencephalon and diencephalon. Unfortunately, however, the effect of the operation is so extremely complex that here, again, the symptoms of chronic abrogation admit only of very general and, therefore, indefinite conclusions. In the first place, the psychophysical activities which continue after removal of the prosencephalon do not afford a safe basis for judgment, since it remains uncertain whether and how far they themselves owe their origin to some compensation of functions. But more than this: it is exceedingly difficult, indeed, in many cases it is impossible, to distinguish between complicated reflexes, that take place without any accompaniment of conscious sensations, and reactions that occur at the incentive of sensations and sense perceptions. Hence the investigator can never obtain more than a negative result. The functions that are permanently abrogated by removal of the prosencephalon are, in all probability, conditioned exclusively upon its integrity. But the activities that are temporarily deranged, and presently restored again, must remain of doubtful significance, since there is no means of determining the extent of possible compensations. Now we saw that birds, rabbits, and even dogs are not only capable, in the decerebrised state, of purposive reaction to tactual and visual stimuli, but also adapt their movements, like normal animals, to external impressions. They avoid obstacles, they recover their equilibrium by balancing, etc.; nay more, they apparently execute spontaneous movements, -- they run to and fro, seize and swallow the food that is offered them, and respond by expressions of pain to intensive sensory stimuli. That is to say, they appear to be in full possession of the sensory and motor functions. On the other hand, they give no signs of intelligence, and never express joy or any other of the complex emotions. Moreover, their spontaneous movements are more uniform and restricted than those of an uninjured animal (See above, p. 262). In a word, these final abrogation phenomena lead us to the general, and, we must also admit, indefinite conclusion that the intelligence, the higher affective processes, and the compound voluntary actions are conditioned upon the integrity of the cerebral hemispheres. We term this result indefinite, first, because the psychological terms that enter into the functional determination require a more exact psychological definition before any precise meaning can he attached to them, and secondly because it is clear, even without any such definition, that an absolute delimitation of the intelligence and the complex affective and volitional processes, as contrasted with the lower processes of the same kind that may possibly continue after the removal of the prosencephalon, is a matter of extreme difficulty. In any event, however, the [p. 285] distinction, so far as it is practicable at all, must also be left over for the detailed psychological analysis of the processes involved.
(c) -- Results from Comparative Anatomy and Anthropology
The general conclusion to be drawn from the abrogation phenomena, that the physiological function of the cerebral hemispheres stands in intimate relation to the intellectual activities and to the complex affective and volitional processes, is, upon the whole, confirmed by the results of comparative anatomy, evolutionary biology and anthropology. Comparative anatomy shows that the mass of the cerebral lobes, and more especially their superficial ridging by fissures and gyres, increase with increasing
(d) -- The Hypotheses of Localisation and their Opponents. The Old and the New Phrenologies
These obvious differences in the superficial configuration of the cerebral hemispheres naturally suggest the hypothesis that the general connexion between brain development and mental endowment, which appears in them, is paralleled by specific relations between the relative development of various parts of the brain surface and definite directions of mental capacity. This hypothesis, which in itself is entirely justified, forms the point of departure of the system of 'phrenology' founded by FRANZ JOSEPH GALL. Unfortunately, however, the physiological and psychological premisses upon which GALL worked out his ideas are untenable, and the observations themselves and the conclusions drawn from them betray lack of accuracy and scientific caution. GALL regarded the mental functions as the business of a number of internal senses, to each of which, on the analogy of the external senses, be attributed a special organ. Nearly all of these internal sense organs he localised on the outer surface of the brain, assuming a parallelism of skull-form and brain-form which, as can easily be demonstrated, does not obtain, at any rate to the extent required. GALL distinguished twenty seven 'internal senses,' in naming which he makes use at need of the expressions sense, instinct, talent, and even memory: we find, e.g., sense of place, sense of language, sense of colour, instinct of propagation, instinct of self-defence, poetic talent, esprit caustique, esprit métaphysique, memory of things, memory of words, sense of facts, sense of comparison, etc. It is useless to repeat the statements of the phrenologists regarding these localisations. It may, however, be mentioned that in one case -- and the fact shows that he possessed some gift of observation -- GALL made a lucky hit: he localised his 'sense of language' in a region of the cerebral cortex approximately corresponding to the area whose lesions, as we shall see later on, have been proved in modern times to constitute the most frequent cause of the syndrome of 'aphasia.' Indeed, the discovery of the seat of aphasia is directly traceable to GALL'S suggestion, as has been expressly acknowledged by BOUILLAUD, to whom it is due. At the same time, we must not forget that even in this instance, where a pronouncement of GALL'S has received a certain measure of confirmation from the facts, there is really an essential difference between what was actually discovered, viz., the anatomical seat of central derangements of speech, and the phrenological 'organ of language.' The two can, in truth, be identified only if we force a phrenological interpretation upon the phenomena of [p. 288] aphasia, which in the light of actual analysis they will not fear. Cf. below, § 7b.
When physiology first took the field against the phrenological doctrine of localisation, it was itself but poorly armed for the combat. It was inclined to lay a disproportionate weight upon the indefinite or equivocal results of extirpation experiments on animals. It was easily influenced by false analogies, and did not hesitate to accept psychological theories that at bottom were no less questionable than the 'internal senses' of the phrenologists. Thus FLOURENS, whose views long held undisputed sway in physiology, insisted strenuously upon the unity and indivisibility of the cerebral functions, and argued from it that their organ must be similarly indivisible. This opinion was largely determined by the analogy of other, unitarily functioning organs. Cerebellum, oblongata and myel were each endowed by FLOURENS with an independent and specific function, discharged by the organ as a whole. In the same way, the mass of the cerebral hemispheres stood for him upon a single physiological level, like the substance of a secreting gland, e.g. the kidney. He found confirmation of this view in the observations on the results of partial and total extirpation of the prosencephalon in animals that we have discussed above: for these experiments show, in general, that partial removal of the cerebral lobes simply weakens the mental functions, as a whole, and does not, as on the hypothesis of a localisation of functions might be expected, abrogate certain activities and leave the rest unimpaired; while total extirpation of the cerebrum completely destroys all spontaneous expressions of the mental life, i.e., as FLOURENS phrased it, all 'intelligence and will.'
This doctrine, of the specifically psychical function of the prosencephalon and of the indivisibility of that function, presently became untenable. It was overthrown partly by the pathological observations on the consequences of local lesion in man, partly by the growth of knowledge regarding the structure of the brain and the course of the conduction paths. Its place was taken by the modern theories of localisation, which to a certain extent attempt a rapprochement with the doctrine of phrenology. At the same time, they mark a twofold advance beyond GALL and his disciples. First, on the side of physiology, GALL'S 'mental organs' are replaced by the idea of the separate 'centres,' correlated with peripheral spheres of function that are, upon the whole, definite and clearly distinguishable -- the sense organs, the various muscular territories, etc. This change of view is evidently a reactive effect of the advance in knowledge of the anatomy of the conduction paths. Secondly, on the side of psychology, such monstrous terms as sense of facts, reverence, philoprogenitiveness, or -- to take instance of another sort -- sense of language, poetic talent, etc., are banished from [p. 289] the list of localisations, and the terms 'sensation and idea,' terms which, as the framers of the theories believed, represent the two fundamental forms of psychical process, and retained in their stead. 'Sensation' means, in this connexion, any conscious reaction evoked by external sensory stimuli, while 'idea' includes, in accordance with the nomenclature of the older psychology, all kinds of 'memory image.' >From these general premisses, the doctrine of localisation or, as its close relations to the older phrenological theories justify us in calling it, 'modern phrenology' has developed in two directions. In both forms, it is based upon the assumption that the cerebral cortex is divided up into a number of sensory centres, in which the excitations brought in along the sensory conduction paths release the specific sensations. The centromotor regions are counted among these sensory centres, on the further assumption that a voluntary action may be adequately defined as the connexion of some reflex, arising either in lower centres or in the cortex itself, with a concomitant sensation of touch or movement. As we leave this common ground of general theory, however, opinions begin to diverge. The one form of the doctrine of localisation asserts that the centres of sensation and idea are strictly connected, so that every sensory centre covers both processes, and the entire cortical surface is therefore essentially composed simply of a number of adjacent sensory centres. The distinction of sensational and ideational functions within each centre is then conditioned solely upon the action of determinate -- functionally, not morphologically discriminable -- elements. It thus becomes necessary to posit the existence of two sorts of cortical cells: sensation cells and idea cells. The former are supposed to receive peripheral excitations by direct conduction; the latter take up excitations proceeding from the sensation cells, and thereby acquire the capacity of renewing these excitations, -- a process that, for brevity's sake, has also been termed the 'deposition' of ideas in the specific memory cells. This form of localisation theory, which we may call the 'pure sense-centre theory,' was first worked out by MEYNERT on a morphological basis, and has since been employed by H. MUNK for the interpretation of his experiments on animals. It is widely current at the present day, both in physiology and in pathology,. The second form of the localisation doctrine differs from the first mainly by its assertion that the central areas which subserve the colligation of sensations, and therefore also the retention of ideas, the centres, in fact, which underlie the complex psychical functions at large, are spatially separate from the sensory centres, though connected with them by manifold systems of association fibres. It accordingly gives this second order of centres, whose office it is to colligate the various sense departments, the special name of 'association centres'; and we may therefore designate the second form of the localisation theory, briefly, as the 'theory of association centres.' According to this view, [p. 290] the essential activity of the cerebrum consists in the function of the association centres, while the sensory centres serve, on the whole, simply to take up the sense impressions, in the order in which they affect the peripheral organs, and to raise them to consciousness by their projection upon determinate cerebral surfaces. The expression 'association centre,' is used, in this connexion, both in a physiological and in a psychological sense: physiologically, it is the 'association fibres,' characteristic of these centres as such, that are connected only indirectly, viz., by way of the sensory centres with which they are correlated, with the periphery of the body; psychologically, these association centres are looked upon as the substrate of the associative processes upon which, as the psychology of association teaches, and as our theorists believe, all the higher psychical functions depend. This second form of the localisation doctrine is, unquestionably, superior to the first, in that it leaves a somewhat freer scope to hypotheses of, the origin of the more complex psychical processes; the schematic antithesis of sensation cells and idea cells, which naturally leads to a corresponding and correspondingly untenable classification of the psychical processes themselves, is replaced by the broader antithesis of direct sensory excitations and associations. At the same time, however, the idea of association and of the association centre is left very indefinite. It is, in the last resort, bound down to the belief that cortical areas which stand in exclusive connexion with association fibre systems are the vehicle of the more complex psychical activities; so that, from the functional point of view, the expression 'psychical centres' would really be the more correct. But such a term shows very plainly that the second form of the localisation doctrine brings us perilously near, once more, to the doctrines of the older phrenology; and that, if it does not run altogether on the old lines, this is principally due to the praiseworthy caution of its representatives, who have so far refrained from correlating the different cortical areas of the association theory with complex psychical activities or endowments of a definite kind.
These modern hypotheses of localisation, like the old-time phrenology of GALL, have not been allowed to pass unchallenged from the side of physiological observation. GOLTZ and his pupils, in particular, have disputed the strict localisation of the psychical functions, on the ground of the defects observed after partial extirpations of the cortex. In opposition to the theory of sharply circumscribed sensory centres, these investigators insist upon the complex character of the disturbances following from local lesions, and upon the general reduction of the intellectual functions after partial removal of the cerebral lobes. They thus come back to a view which resembles that of FLOURENS: they emphasise the necessity of the co-operation of the different cortical regions in the psychical functions, though they have given up the hypothesis of the functional equivalence of all the parts of the cere-[p. 291]brum, as untenable in the present state of our anatomical and physiological knowledge.
If, now, we attempt an appreciation of these different theories, standing opposed to one another in the doctrine of the cerebral functions, we must, of course, try to do justice to all the departments of experience that our judgment involves: to anatomy and pathology, that is, not less than to physiology and psychology: and we must be the more careful, since the quarrels between the older and newer localisation theories and their opponents have evidently been due, in no small measure, to the all too common tendency to rely, exclusively and onesidedly, either upon the anatomical facts or upon the results of experiments with animals. In both events, we may add, psychology has usually been regarded as an unclaimed territory, with which either side might deal as seemed best to it. We will ourselves, therefore, begin by setting psychology in the foreground, though we shall, as was said just now, attempt an impartial treatment of the other sciences also. >From the psychological point of view, then, the sense-centre theory must be pronounced untenable; neither of its constituent hypotheses can be accepted. In the first place, the 'sensory centres,' as is clear from the results of psychophysical analysis of the functions of perception and from pathological observations on man, are not simple repetitions of the peripheral sensory surfaces, but are in the strictest sense of the word 'centres,' that is, areas in which the different peripheral functions concerned in the activities of sense are centralised. Thus, the visual centres bring together the functions of visual sensation, of energy and synergy of movement in the visual organs, of the relations of these processes to the visual reflexes that run their course in lower centres, and so on. The sensory centres would not be centres at all, but superfluous duplications of the peripheral organs, if they possessed no other significance than that of repeating the excitations touched off at the periphery. In this regard the view embodied in the sense-centre theory is the result partly of an inadequate and prejudiced reading of the anatomy of the conduction paths, partly of a wrong interpretation of the experiments on animals, which, as we know, are pre-eminently ambiguous upon the point at issue. In the second place, the theory opposes 'sense cells' to 'idea cells'; excitations are supposed to flow from the former to the latter, and there to be deposited in the form of memory images. It thus transforms a wholly inadmissible psychological distinction into an equally inadmissible physiological hypothesis. The idea that sensations and ideas are absolutely distinct conscious contents belongs to the older spiritualistic psychology, which taught that 'ideas,' as contradistinguished from the sensations evoked by physical stimuli, are purely mental processes, the prerogative of the mind itself. This spiritualistic distinction is, of course, a pure product of metaphysical speculation, and [p. 292] survives at the present time only in a psychology of reflexion that has turned its back once and for all upon the facts of psychical experience. No truly psychological observer will be found to assert to-day that ideas exist independently of sensations, or that the sensations which enter into our ideas differ in any other way than by their intensity, duration, and fragmentary character from the sensations aroused by external sensory stimuli. This distinction, derived as we have said from the spiritualistic psychology, and then clothed about with the garb of materialism, stands in actual fact upon the same level, in psychology as would, in anatomy, the dictum of some present day philosopher that the mind has its seat in the epiphysis.
As compared with the sense-centre theory, the theory of association centres has certain indisputable advantages. It has dissolved the unnatural alliance with the ideas of metaphysical psychology, and has attempted instead to enter into relations with the psychology of association, which agrees better with our modern conceptions. But, on the one hand, it still holds to the erroneous view that the 'sensory centres ' are central repetitions of the peripheral sensory surfaces, which latter must be projected upon the brain cortex solely that they may be brought into touch with the consciousness which resides there; and, on the other, it confuses in a very dangerous way the purely anatomical idea of 'association fibres,' that connect different regions of the cerebral cortex with one another, and the psychological idea of association. We are forbidden to suppose that the association fibres are, so to say, the vehicles for the production of associations of ideas, by the simple fact that the commonest and most important associations are those obtaining between the sensation elements of one and the same sense department. Hence the paths that run between different sensory centres could, at most, be taken as the conjectural substrate only of what are called 'complications,' i.e. of associations between disparate ideational elements. But if the idea of 'association fibres,' understood in this restricted sense, would possess a fairly definite meaning, the same thing can hardly be said of the idea of an 'association centre.' Are we to imagine that the association fibres running between the various sensory centres are inadequate to mediate complications, and that the independent function of an organ that receives association fibres, and association fibres alone, is further necessary to their production? This is the obvious meaning to be put upon the term 'association centre'; but it is scarcely the meaning that really attaches to it. It seems rather that the theory is here influenced by reminiscence of the psychology of association, very much as the sense-centre theory, in its distinction between sensations and ideas, is dominated by the old spiritualistic view of mind. The psychology of association seeks, as we know, to derive concepts, judgments, complex intellectual and affective processes all and sundry,[p. 293] from associations of ideas. And the theory of association centres is evidently inspired, in the last resort, by the idea that the various complex psychical products are originated in these centres, -- always, of course, with the closest co-operation of the sensory centres with which they are connected by association fibres. Here, then, we have a justification for the expression 'psychical centres.' The further the distinction of such centres is carried, however, the more nearly do we approach to the 'internal senses' of the older phrenology. For the psychical functions ascribed to them must, naturally, become more special, and therefore more complicated, in proportion as they themselves increase in number.
This extreme subdivision of the psychical functions is opposed by the anti-localisation school of modern experimental physiologists. Its representatives are undoubtedly right in insisting upon the multiplicity of relations in which these functions stand, and in affirming, as a consequence, that we cannot speak of sensory centres, in the sense of a definitely circumscribed repetition of the peripheral sensory surfaces, or of psychical centres, in the sense of circumscribed seats of separate mental activities. Indeed, as the hypothesis of the functional equivalence of all parts of the prosencephalon has gradually fallen into disrepute, -- an hypothesis, it will be remembered, which derives in the first instance from FLOURENS, and was revived in the early stages of reaction, -- this principle of functional interaction has come to be our most valuable guide in the psychophysical analysis of the cerebral functions. The new anti-phrenological movement, like its predecessor, sets out from the results of experiments on animals. These abrogation experiments are, however, hardly qualified to lead up to any exact formulation of the principle: their outcome is indefinite and ambiguous, and they are seriously complicated by the effects of vicarious functioning, whose influence is in most cases greatly underestimated. What we rather need is, evidently, an analysis of the individual central functions, in the light of observations of pathological defects, carefully collected and compared. Hence instead of asking: What are the consequences of the lack of a given cortical area, and what functions are accordingly to be ascribed to it? we must now raise the question: What central changes do we find, when a given function (language, the act of vision, etc.) is deranged, and what is the nature of the parallelism between the functional and anatomical disturbances? The great advance that modern pathology, in particular, has made in this field may be attributed, without hesitation, to the fact that it has been forced, by the nature of its problems, to give up the first form of enquiry for the second. And the significance of the advance, for our knowledge of the central functions, lies in the further fact that the first form directs the attention onesidedly, from the very beginning, to a fixed and definite central area, while the second points at once to connexions with other areas and, in general,[p. 294] emphasises the principle of functional analysis as against the former centre of interest, the correlation of determinate functions with determinate parts of the brain. This change of standpoint means a breaking down of the barriers, not only between the different regions of the cerebral hemispheres, but eve, to a certain extent, between the prosencephalon and the posterior brain divisions (more especially the diencephalon and mesencephalon) as well. For the complex functions prove, as a rule, to be functions in which all these departments of the brain are variously involved; so that it is about as sensible to localise a complex function in a restricted area of the cerebral cortex as it would be to throw the sole responsiblity for the movements of walking upon the knee joint, because they cannot be duly performed if that joint is ankylosed. In fine, the analysis of the complex functions themselves comes up as a further problem, whose solution will effectively supplement, at the same time that it transcends, the physiology of the central hemispheres. The solution, as things are, must, it is true, remain imperfect; there are but few functions, at the present time, that admit at all of this sort of analysis. Of those that do, the chief are the central act of vision, the functions of speech, and the processes of apperception.
The problem of the localisation of the psychical functions begins with the great anatomists of the sixteenth century. Among them, VASALIUS was especially instrumental in spreading the opinion that the brain is the seat of the mental activities. For a long time, however, the old doctrine of ARISTOTLE and GALEN, that made the heart the general centre of sensation, held its own alongside of the newer teaching. DESCARTES was the first to regard the brain as an organ subserving the interaction between mind and body. It is with DESCARTES, consequently, that a question arises which was destined thenceforth to play a great part in the discussions of physiologists and philosophers: the question of the seat of the mind. DESCARTES himself, in answering it, made the curious mistake of selecting the epiphysis, a structure which is probably a vestige of the old parietal eye of the vertebrates, and does not properly belong to the brain at all. At the same time, increasing efforts were made, especially in the anatomy and physiology of the eighteenth century, to ascertain the significance of the various parts of the brain. Interpretations were based, as a rule, upon the results of anatomical dissection, though the psychological ideas prevailing at the moment were also of some influence. Thus, at a later time, the mental faculties of WOLFF'S school, perception, memory, imagination, etc., were commonly chosen for localisation, -- which was arbitrary and, of course, very differently worked out by different authors. It is the service of HALLER, in particular, to have paved the way for a less artificial view, holding closely to the data of physiological observation. The reform is intimately connected with his doctrine of irritability, whose chief significance lay in the fact that it referred the capacities of sensation and movement to different kinds of tissue; the former to the nerves, the latter to the muscles and other contractile elements.[p. 295] The source of these capacities HALLER finds in the brain. This organ is connected with the mind and the psychical functions only in so far as it is the sensorium commune, or the place where all activities of sense are exercised and whence all muscular movements take their origin. The sensorium extends over the whole substance of cerebrum and cerebellum. It is therefore certain that every nerve receives its physiological properties from a definite central region; that. i.e., as is also attested by pathological observation, sight, hearing, taste, etc., have their seat somewhere in the brain. At the same time, the conditions of origin of the nerves seem to show that this seat is not sharply circumscribed, but as a general rule is spread over a considerable area of the brain. To the commissural fibres HALLER assigns the function of mediating the vicarious function of sound for diseased parts. He deduces the inexcitability of the brain substance from the fact that the nerve fibres lose their sensitivity, in proportion as they split up within it into finer and finer branches.
The position thus won was never lost by physiology. Nevertheless, the endeavours after a physiological localisation of the mental faculties were constantly renewed, the usual starting point, now as before,, being furnished by anatomy. The views of the reactionaries were systematised by GALL, and in this form long continued to exert an influence upon the science. GALL, it should be remembered, did much real service in his investigations of the structure of the brain. The phrenology  which he founded proceeds upon the assumption that the brain consists of internal organs, analogous to the external organs of sense. As the latter mediate our perception of the outer world, so do the former mediate what may be called a perception of the inner man. The individual capacities localised in the brain were, accordingly, also termed 'internal' senses. GALL distinguished twenty-seven; his pupil SPURZHEIM increased the number to thirty-five. The mental faculties that are ordinarily recognised, such as understanding, reason, will, etc., have no place in the phrenological list. These fundamental forces of the mind are, in GALL'S opinion, not localised, but are uniformly operative in the function of all the cerebral organs, and even in that of the external organs of sense. Every one of these organs is, therefore, as he puts it, an "individual intelligence." His main argument for the analogy of the 'internal senses' with the external sense organs is derived from anatomical investigations; as every sensory nerve is a bundle of nerve fibres, so is the whole brain a collection of nerve bundles. When, now, GALL and his followers came to put these theories into practice, they substituted 'skull' for 'brain': the form of the skull was to yield up information regarding the development of the individual organs. And this means, of course, that they intended, so far as possible, to localise the organs on the surface of the brain. Here, then, at the very outset, is evidence of a tendency to adapt observation [p. 296] to preconceived opinions: a tendency that crops up again in all the special investigations, and robs their 'results' of any sort of value. This apart, however, the monstrosity of the psychological and physiological ideas that underlie the teaching of phrenology marks a long step backward from the more enlightened position occupied by HALLER. The great Swiss already has an inkling of the true principle, that the peripheral organs of the body must in some way be represented and brought into mutual connexion in the central organs. The phrenologists make the brain an independent group of organs, for which they posit specific energies of the most complicated kind.
The fact remains that in one of his localisations, that of the 'sense of language,' GALL hit upon the right path, and in spite of all his errors, this fact has led certain authors in modern times, not only to seek a just recognition of his services to anatomy, -- which are undeniable, -- but also to attempt in some sort a rehabilitation of his phrenological doctrines. It must, however, be remembered, on the other side, that even as regards the syndrome of aphasia the expression "organ of the sense of language" is inadmissible. We can imagine, if we are called upon to do so, that the conduction paths concerned in the function of speech run their course in these particular regions of the brain. We cannot possibly imagine, from what we know either of the brain or of the psychical processes, that a definitely circumscribed brain area is the seat of linguistic endowment, in the same sort of way that eye and ear are organs or the reception of light and sound stimuli, or the pyramidal paths lines of motor conduction. And even so, the faculty of speech is one of the comparatively simple 'internal' senses. How, then, are we to conceive of the mechanism by which the fear of God, philoprogenitiveness, the sense of facts, the impulse of self-preservation, and things like these are localised somewhere in the brain? Granted that GALL was, in his day and generation, one of the highest authorities on brain morphology: this honour is his, and is not to be taken from him. The phrenological system, nevertheless, is and remains a scientific aberration, the joint product -- like its predecessor, the physiognomics of LAVATER - -of charlatanism and unreasoning caprice. For this disease there is no remedy. Hence it is a questionable experiment to rehabilitate any other of the 'mental organs' that GALL pretends to have discovered, as P. J. MÖBIUS has undertaken to do in the interests of the 'organ of mathematical ability.' Looked at as it stands, 'mathematical talent' comes perilously near the psychological monstrosity of GALL'S other localisations. But besides, the unusually marked development of the superior external orbital angle, which MÖBIUS has found confirmed in the case of three hundred mathematicians, admits of two different interpretations. In the first place, it may be -- and this is, perhaps, the more probable hypothesis -- a reactive effect of the mimetic tension of the muscles of the forehead, observable in profound thought, upon the bony skeleton of the face. Or again, it may be due to the fact that, in all highly developed brains alike, the frontal lobes are characterised by their mass and the number of their fissures, quite apart from the question whether or not the intellectual endowment of their owners takes the precise direction of a talent for mathematics. In other words: it must first be shown that the protuberance under discussion is not to be discovered in highly developed brains of poets, philosophers, philo-[p. 297]logians, etc., who at the same time were conspicuously lacking in mathematical ability. So far, this proof is not forthcoming. But suppose that it were given: what would follow? Certainly not, that there was a mathematical organ, in the sense of the phrenologists, but at best this: that we were in presence of a fact, which for the time being we could not explain, and which had about as much value for science as the law that most great men possess unusually large skulls. The principal opponents of the phrenology of GALL and his followers, in the first half of the nineteenth century, were the French experimental physiologists, MAGENDIE and FLOURENS. The views which these investigators developed of the significance of the central organs plainly represent a reaction against the phrenological doctrines. In MAGENDIE this spirit shows itself in the strict congruity of theory with observed facts. FLOURENS had the more decisive influence upon the physiological ideas of the following period. His researches extended to the oblongata, quadrigemina, cerebellum and cerebrum. The first of these he determined as the centre for cardiac and respiratory movements: the quadrigemina as central organs for the sense of sight; the cerebellum as the centre of conduction of voluntary movements, and the cerebral lobes as the seat of intelligence and will. He found, however, that different parts behaved differently, as regards the functions dependent on them. The central properties of the oblongata are confined within a small area, his noeud vital, destruction of which means instantaneous death. The higher central regions, on the other hand, discharge the functions assigned to them uniformly throughout their entire substance. This conclusion is an inference from the fact that the disturbances set up by partial removal of the cerebral lobes, cerebellum or quadrigemina, are gradually compensated as time goes on. It follows, therefore, that the smallest fragment of these organs can function for the whole. In his view of the cerebral hemispheres as the properly psychical centres, FLOURENS was evidently influenced, at the same time, by the traditions of the Cartesian philosophy. DESCARTES had emphasised the indivisibility of the psychical functions, more especially of the intelligence and will, and had on this very account demanded an unitary 'seat of mind.' As DESCARTES' choice of the epiphysis could not be sustained, in the face of more recent experience, FLOURENS substituted for it the total mass of the cerebral hemispheres. FLOURENS doctrines thus came, in virtue of their spiritualistic prominence, into sharp conflict with the more materialistically coloured ideas of the phrenologists: a circumstance that was not without import for the issue of the struggle.
Their acceptance in scientific circles was, however, chiefly due to he fact that they gave a fairly accurate representation of the observational data in experiments on animals. It was not realised that the same psychological difficulties attach to them as to the phrenological theory of organs. Nevertheless, intelligence and will are also complex capacities. That they should have their seat in any the least fragment of the cerebral lobes is, after all, just as difficult of comprehension as that memory for languages, sense of place, etc., should [p. 298] be somewhere localised. Moreover, it remains an open question what significance is to be ascribed to the separate parts distinguished by anatomical dissection of the cerebral hemispheres, if they are throughout as uniform in functional regard as, say, the liver.
So it came about, even before the dawn of the new era of localisation, that the anatomists, where they ventured at all upon speculation concerning the significance of the different parts of the brain, were apt to recur, urged doubtless by arguments derived from their own science, to the idea of localisation of particular mental capacities. And as, in course of time, the connexion of anatomical, physiological and pathological observations became more intricate, the views introduced into physiology by FLOURENS gradually lost their hold upon men's minds. The determining factors in the change of opinion were two: first, the investigations into the elementary structure of the central organs, and, secondly, the physiological and pathological experiences regarding the localisation of certain sensory functions and motor effects. Epoch-making, in the latter connexion, was the renewal of interest by BROCA in the observations made long since by BOUILLAUD on the anatomical substrate of aphasia. Still, a certain contradiction remained between these results and the consequences of partial removal of the hemispheres by FLOURENS' method. The permanent symptoms produced by the latter operation consisted, not in the abrogation of particular functions, but in the weakening of all: a fact verified in the most recent times by GOLTZ. So, in the controversy carried on principally between H. MUNK and GOLTZ, we have repeated, in modern terms and within the limits of experimental physiology itself, the older issue between FLOURENS and the phrenologists. But the new phrenological view, as represented by the 'sense-centre theory' and the 'theory of psychical centres' discussed in the text, is indefinitely better fitted to make terms with science than was the old phrenology; and the newer antiphrenologists, in the same way, have given up their original and impossible assumption of the functional equivalence of the different parts of the brain, and lay increasing emphasis upon the principle of regional interaction in the various complex functions. This principle, now, taken together with the manifold phenomena of substitutive and auxiliary function, leads inevitably to the idea of a relative localisation of functions. We say 'relative' for two reasons: first, because it is never the complex functions themselves, but only their elements, that are localised; and, secondly, because these elementary functions may also suffer all sorts of shifts and changes as a result of the processes of vicarious function.
§ 7. Illustrations of the Psychophysical Analysis of Complex Cerebral Functions
(a) -- The Visual Centres
Of all the sensory nerve conductions, it is those of the acusticus and opticus, shown schematiclly in Fig. 77 and 78 (pp. 183, 186),that, by the complicated course of their paths and the number of central areas which these [p. 299] paths unite, make the most insistent demand for physiological analysis. In the case of audition, however, we have not the data necessary for the functional interpretation of the various areas involved in the nervous conduction. The connexions with certain sensorimotor and regulatory centres, in particular, -- centres like the pregemina, cerebellum, etc., -- can, in the present state of our knowledge, be referred only quite generally to the interactions between auditory impressions and rhythmical movements. The central mechanism of these movements themselves is still wrapped in so much obscurity that the general notion of interaction cannot be replaced by any more definite ideas. In the case of vision, the conditions are more favourable: not so much, perhaps, because the investigation of its anatomical substrate has been brought to greater completion, as because the physiological and psychological analysis of the functions as such has been carried further.
Anatomically and physiologically, the act of vision is, in the first instance, characterised by the fact -- unparalleled, save in the one instance of olfactory sensations -- that it is, from the very beginning, in some sort a central process. The retina, as we have seen, represents a brain area displaced to the periphery of the body. Hence it is by no means to be regarded as a simple receiving apparatus for external light stimuli, but rather as an organ of complicated structure, containing not only nerve terminations, but also manifold ganglionic formations that mediate connexion with other and higher nerve centres. The single elementary process that, in a certain sense, reaches its conclusion in this peripheral organ is, we must suppose, that transformation of the external light vibrations into some kind of photochemical process, the physical correlate of which we find in the sensations of light and colour. In all probability, the vehicles of this transformation are the specific sensory cells of the retina, the rods and cones (s, z Fig. 78).
It is at this point that the process of vision proper begins. The photochemical changes have simply prepared the way for it, by impressing their peculiar qualitative differentia upon the excitations conveyed along the opticus paths. The process of vision itself is compounded from the manifold connexions into which these primary optical excitations enter, and by which they obtain their concrete contents. This latter always carries with it a number of secondary excitations, which are the principal factors in giving the particular optical stimulus its relations to other sensations, and thus mediating the localisation and spatial arrangement of light impressions. It is clear that the scheme of a simple and direct connexion between every point upon the retina and a corresponding point in the visual centre of the occipital cortex -- a schema still to be found here and there in the physiologies -- is not able to satisfy these conditions. How inadequate it is will be [p. 300] seen at once from a consideration of Fig. 78 (p. 186), although the Figure does no more than indicate the course of the simplest conduction paths those that admit of relatively certain interpretation, and altogether omits such others as the pupillary reflex, which plays an important part in adaptation to brightness, and the branch conduction to the cerebellum, the function of which has not yet been definitely determined. The nearest approach to the original simple schema seems to be made by the direct optic radiation ss. But even this path is interrupted, as it passes through the thalamic region (AK), by nodal points, at which, in all probability, connexions are also made with other paths. Further: as we said above (pp. 188, 230 ff.), the decussation of the optic nerves occasions a partial transposition of the paths of the right and left sides: the rearrangement is adapted to the functions of binocular vision, but at the same time suggests a relation to the centrifugal motor innervations proceeding from the occipital cortex. This relation is also attested by the structure of the visual cortex (p. 219), and by the probable existence of centrifugal conductions issuing from it (c'f' Fig. 78). Again, a second main conduction of the opticus paths, which, if we may judge from the position of the chiasma, is undoubtedly involved in the same plan of decussation that includes the principal path, leads to the mesencephalic region, where it enters, in the pregeminum (OV) into two connexions. The first of these is a reflex connexion to the nidi of the oculomotor nerves (rr). It is, probably, at once same-sided and crossed, according to the way in which the fibres are distributed in the chiasma, and the conditions laid down by the requirements of binocular (panoramic or stereoscopic) vision. The second is a sensory connexion; for the supposedly centrifugal terminations of the opticus in the retina (e f) also take their origin from the mesencephalic region. The excitations carried by this path may be brought in either along the centripetal path cp that ends in the same region of the brain, or along the higher centrifugal path c'f' that reaches it from the visual centre. The functional arrangements warrant the conjecture that, in both cases, the paths will take a crossed course; for it is but natural to regard the centrifugal system cf, c'f' as the substrate of the coexcitations, which can also be demonstrated psychologically as concomitant sensations, and which in all probability have a part to play in the functions of binocular vision. In view of the motor synergies which these functions engage, and of their dependence upon the sensory excitations of the visual centre, the central portion c'f' of this centrifugal path will presumably make connexion, not only with its sensory continuation cf, but also with the motor path rr, so that the regulation of ocular movements by light impressions can be effected in two ways: the reflex, by direct release from cp, and the centromotor, by the excitations brought up from the visual cortex in c'f'.
Now we must, of course, assume, as a general rule, that every excitation [p. 301] of the peripheral organ of vision, whatever point it may strike, will discharge at one and the same time into all the different paths of conduction opening up before it, provided always that none of these paths have been rendered impassable by interruptions of conduction. In the entire complex of organs that thus work together in the particular act of vision, the retina on the one side and the visual centre of the occipital cortex on the other constitute the two principal centres. Their functions are in some sort antithetical. The retina, lying farthest out towards the periphery, plays the leading part in the origination of sensations; the visual cortex, lying farthest in towards the centre, plays this part in the final combination of the separate functional components of the act of vision. The transformation of light vibrations into photochemical processes, which takes place in the elements of the retina, is at any rate indispensable for the first origination of light sensations: for observations made on the congenitally blind prove that the brain cannot mediate such sensations unless the retina has previously been in function. On the other hand, the visual functions, once originated, may persist after removal of the external sense organ; the victim of accidental blindness, despite the atrophy of his optic nerves, if he originally possessed any vivid sense of colour at all, sees coloured memory images and, more especially, can enjoy a wealth of colour in his dreams. We must accordingly suppose that the excitatory processes in the central apparatus, particularly in those of the occipital visual centre, come in course of time, through the influence which they exert upon the processes of external stimulation, themselves to resemble those processes. The change is an illustration of the great adapt ability of the central nervous substance to the varying conditions of excitation, vouched for by many other facts. This capacity for adaptation is, indeed, evidenced by the central process of vision in two different ways: intensively, in the change just mentioned, and extensively, in the manifold functional substitutions that follow the loss of particular central parts. Apart from minor phenomena of this kind, which appear in cases of central lesion and are probably to be referred to the vicarious function of neighbouring parts of the same brain area, e.g. of particular divisions of the visual cortex, one for another, we have here to consider two principal substitutions, again of very different character, that occur between the two main departments of the optic conduction, the mesencephalic and prosencephalic regions. Both of these regions bring together sensory and motor conductions that belong to the same peripheral organs; so that, in the nature of the case, loss of either centre is compatible with the retention of certain essential visual functions, and a new course of practice may partially make good the defect. Observation, as we saw above, proves that these possibilities are realised. The visual centre in the mesencephalon is able, in particular, to discharge the most essential of the visual functions, independently of the [p. 302] visual centre in the occipital cortex (pp. 260 ff.). True, the defects that remain, even with the utmost extent of vicarious activity, demonstrate at the same time that under normal circumstances the act of vision is a complex function, conditioned upon the co-operation of all these centres, whose several functions are themselves of a complex nature. In this analysis of the central functions of vision, we have still left out of account two principal factors, whose significance cannot at the present time be estimated in any sort of detail, though a rough guess may be made at their meaning. The first consists of the relations to the motor regulatory mechanisms situated in the cerebellum; the second of the connexions mediated by the association systems of the cerebral cortex both with other sensory centres and with yet more central brain regions, which are not directly correlated with definite sense departments, but themselves contain the junctions of various sensory and motor conduction paths. Now we cannot conceive of an optical excitation that does not, to some degree, release at any rate a certain proportion of these manifold excitations. Hence we might conclude without hesitation, merely from the morphological relations of the optic conduction and the physiological analysis of its processes, that the simplest act of vision is, physiologically, an occurrence of great complexity, even if we were not constrained to posit this complication of conditions by our psychological analysis of the visual processes.
(b) -- The Speech Centres
The name 'speech region' is ordinarily applied to a cortical area, lesions of which, whether they affect larger or smaller portions of its substance, are attended by disturbances of the functions of speech, without setting up at the same time other psychical disturbances, especially those of what is termed 'the intelligence,' of any noticeable kind. Where the intelligence is affected, there is always good reason to refer its impairment to more extensive changes involving other cortical areas. Psychical disturbances of this sort, due to diffuse cerebral disorder, may be accompanied by derangement of speech, or even by complete abrogation of the speech functions, without direct injury to the speech centre itself. In such cases, the disturbances of speech are evidently secondary symptoms. Hence the only phenomena that are important for the relations of speech to definite cortical areas are those observed when the seat of injury is strictly confined to the speech centre proper. This region, in contradistinction to the sensory centres, whose representation in the brain cortex is without exception bilateral, has the peculiarity that its development is exclusively unilateral. Since the majority of mankind are right-handed, it is situated, as a result of the decussation [p. 303] of the conduction paths, upon the left hemisphere. The corresponding cortical area of the opposite side is not employed for any other function. Under normal circumstances it serves, we may suppose, though for the most part to a very limited extent, as an auxiliary to the principal speech centre. If, however, the function of the latter is abrogated, it enters under stress of the new conditions upon a special course of practice, and gradually takes the place of the lost organ. This appears, at any rate, to be the only way in which we can explain the restoration of function observed in cases where the speech centre of the left side has been destroyed over a wide extent of the cerebral surface.
According to pathological observations of the phenomena that result from its partial abrogation, the entire speech region divides into several sub-regions or 'speech centres,' as they are called, each of which would seem to preside over a definite phase of
Somewhat less assured is the localisation of two further functions, functions that do not necessarily belong to speech in the stricter sense of the word, but that are intimately connected with it: the functions of writing, which, as a predominantly motor activity, connects with the articulation of sounds, and the functions of reading, which, as a more sensory process, connects in the first instance with the auditory perception of words. It has often been observed that abrogation of the movements of writing, with retention of the capacity of voluntary contraction of the muscles concerned, may be produced by lesion of a cortical area lying directly above the motor centre and belonging to the medifrontal gyre (S Fig. 102). This syndrome has been termed 'agraphia.' It seldom occurs, apparently, in pure form, but either accompanies motor aphasia or is connected with disturbances of the other voluntary movements of hand and fingers. A comparison with the position of the general motor centres (Fig. 88, p. 205) shows, also, that these and the centres for the special functions of speech and writing are either entirely coincident or, where that is not the case, directly apposed. As regards reading, we find that the relation of the optic to the acoustic speech centre corresponds to a certain extent with that of the writing centre [p. 305] to the centre for articulation. It appears, from numerous observations, that the optic speech centre belongs to a region of the subparietal and the second occipital gyres (the 'angular gyre': O Fig. 102) situated between the general centre for vision and the acoustic speech centre. Destruction of this region produces the peculiar syndrome of 'alexia' or 'word blindness': words can be spoken, and can also be heard, understood and remembered; but their written or printed symbols are not understood: they appear as meaningless pictures: although in other respects the visual functions remain unimpaired. All these disturbances, now, can not only occur in the most various combinations, with each component developed in a different degree, but may also be accompanied by further central disorders; so that it is but seldom that the syndromes of the typical forms of aphasia are seen pure, and unmixed with other phenomena. At the same time, the lesions of the different regions themselves produce somewhat divergent effects, according as the cortex proper or the subcortical parts are more seriously affected. Hence it is customary to distinguish cortical and subcortical disturbances. The latter are also termed intercortical or conductive disturbances, on the assumption that, while the cortical lesions involve the speech centres themselves, the subcortical injuries interrupt the connexions mediated between different centres by association fibres.
These manifold gradations and combinations must here be followed out in some little detail, though only in so far as they furnish the necessary data for an appreciation of the psychophysical aspect of the phenomena, and afford an insight into the peculiar significance of that category of complex 'centres' to which the speech centres belong. An especial interest attaches, in this regard, to the connexions in which the typical cases of 'sensory' and 'motor' aphasia come under observation, and to the phenomena of mutual assistance and gradual recovery that follow in their train. The association paths anatomically demonstrable at all points between the centres marked out in Fig. 102 naturally suggest themselves as the substrate of these connexions. Since, however, nothing more can at present be learned from the anatomical maps than the general possibility of this synergy of the different centres, pathologists are accustomed, in order to explain the connexions in individual cases, to base their discussions upon a geometrical scheme, in which the centres themselves are represented by circles, and the paths of conduction to and between them by single lines of connexion joining the circles. A simple schema of this kind is shown, e.g., in Fig 103. It adopts, in all essential features, the arrangement suggested by LICHTHEIM. The little circles M and S (A) denote the primary, motor [p. 306] and sensory-acoustic, speech centres; E and O are the secondary centres co-ordinated with them, E that of the movements of writing, O that of visual word-pictures. Besides these centres, however, all schematic representations of this sort are compelled to introduce a 'concept centre,' C, and lines of connexion running to it from the primary speech centres M and S (A), in order to indicate the relations with the ideational or conceptual contents of the words. It is obvious that this centre C, together with the name attached to it, is in reality only an indefinite expression for the manifold relations in which the various speech centres must stand with all the cortical areas that can claim a share in the origination of the ideational and affective contents of the constituents of speech. In what follows, we shall, for brevity's sake, include this contents under the single term 'meaning contents': an expression which, in the present instance, recommends itself by its very indefiniteness. It need, again, hardly be pointed out that such a meaning contents cannot possibly be conditioned upon any sharply circumscribed central area, but presupposes the combined activity of variously constituted groups of sensory centres and of many other of the regions that belong to the indeterminate category of 'association centres.' The circle C, therefore, can stand here simply as the indefinite symbol of these manifold relations. This presupposed, the schema first of all explains the occurrence of two general forms of speech derangement. Abrogation of particular functions will occur, Whenever certain of the speech centres themselves, M, S (A), E, etc., are destroyed in whole or in part; interruption of conduction or, in psychological language, abrogation of the associations normally subsisting between the various phases of the speech function will occur, when the connexions between the centres are broken, e.g. at 3, 6, 10, etc. In the latter event, the phenomena will take on a different form according to the direction in which the processes are conducted or, in other words, the associations made. It is, however, generally assumed that the conductions may [p. 307] take any direction as between the centres themselves, and that only the peripheral lines running to the two principal centres M and S convey their impulses towards a predetermined goal. The centre S receives its excitations centripetally from the direct auditory centres and, by way of these, from the peripheral organs of audition. The centre M gives out centrifugal impulses, first of all to the direct motor centres of the brain cortex, and then, from these again -- probably with the co-operation of the co-ordinating and regulatory centres of diencephalon, mesencephalon and cerebellum -- to the organs of articulation. So far, the directions of conduction are opposed. These articulatory movements are, however, accompanied by sensations, sensations of extreme importance for the uninterrupted flow of articulate speech; and we must accordingly posit, as their substrate, the existence of other, centripetal excitations, issuing from the motor organs. The conditions are indicated in Fig. 103 by the two arrows at 4.
From a plan of the speech centres and their connexions, such as is given by this Fig., we can read off without difficulty the different forms of speech derangement and their possible combinations. Thus, destruction of M will produce the syndrome of motor, destruction of S that of sensory aphasia; lesion of E will mean agraphia, lesion of O word-blindness. The schema also shows the more complex symptoms that may result from interruptions of conduction. Suppose, e.g., that connexion is broken at 3 between M and S. Words and phrases will still be heard; and, provided that the conduction from S to C is unimpaired, will be rightly understood. Further, if the conduction between C and M is intact, they may also, in contradistinction to the symptoms of cortical motor aphasia, be spontaneously uttered, with their right meaning upon them. On the other hand, heard words cannot be repeated, or can be repeated only with difficulty, -- perhaps through the mediation of the idea centre, C, -- because the requisite association path between M and S, the path which is supposed to run in the alba of the insula, is now out of function. Similar consequences follow from an interruption of conduction between M and E, S and O, O and E. A break at 12, e.g., would mean that the writing of words to dictation has become impossible, though printed or written words can still be copied from sight, provided that the conduction O E remains uninjured; etc., etc.
Adequate, however, as this schema appears to be, for an understanding of the manifold forms of possible derangement, it nevertheless fails in two respects to do justice to the facts. In the first place, it represents certain disturbances as probable, even as necessary, which in reality do not occur at all, or, if they occur, do so only with very considerable limitations and modifications. In the second place, it leaves a large number of phenomena -- more particularly the qualitative peculiarities of the disturbances, and the compensations due to interaction of the different functions -- alto-[p. 308]gether unexplained. On the former count, it may suffice here to mention the most striking example of incongruity between the anatomical plan and the actual functional conditions. According to the Fig., there may be interruptions of conduction between S and M that will prevent the translation of the heard into the spoken word, while the patient's apprehension of the meaning of words and power of spontaneous word formation remain unimpaired. But phenomena of this nature do not occur at all, in a pure form; the symptoms usually ascribed to the disturbance in question are paraphasic, and appear to be of more complex origin. The incongruity thus facts is evidently due to an erroneous assumption which underlies the construction of the Fig., -- erroneous, because it disregards the actual psychological association of the speech functions: the assumption, namely, that the central areas C, whose activity is necessary for the origination of the meaning contents of a verbal idea, are connected in the same way both with the motor and with the sensory speech centres. Viewed in the light of the normal phenomena of speech associations, this assumption is wholly inadmissible. On the contrary, the possibility of a movement of articulation is so intricately associated with the acoustic word symbol, and the acoustic word symbol itself so frequently precedes articulation as a constituent of the total word complication, that the association may be much more probably referred to the indirect path C S M than to the direct connexion C M. But if the indirect road is really followed, then the syndrome to be expected after interruption of conduction at 3 will, of course, be entirely different: spontaneous articulation and the translation of the heard into the spoken word must always suffer together. And if this conclusion, again, is not borne out in all cases, the result simply proves that in the preparation of the schema functional moments of such weight and importance have been left out of account that we may well doubt whether constructions of the mind can serve any useful purpose.
That these functional moments exert a real influence is shown with especial clearness in two groups of phenomena, whose whole character forbids the relegation of the aphasic symptoms, whatever they may be, to any rigid scheme of localisation. The first group consists in certain qualitative peculiarities, which attach to pathological amnesia in general precisely as they do to the normal lapse of verbal memory in advanced age. They find expression in the law that those words disappear most readily from memory which are associated in consciousness with concrete sensible ideas. The amnesic symptom that sets in most easily, and that is therefore the first to [p. 309] appear in old age, is, accordingly, forgetfulness of proper names. Next in order come the ideas of concrete objects: chair, house, table, etc. Somewhat more durable are the concrete verbs: go, stand, cut, strike, etc.; still more the abstract ideas, nominal and verbal: virtue, love, hate, have, be, become, etc. Finally, as most permanent of all, come interjectory words and the abstract particles: but, for, and, because, etc. It is surely evident that no anatomical schema, however complicated its construction, can do justice to this sequence of phenomena. On the other hand, the order of impairment is explained at once by the associations in which verbal ideas are uniformly involved in consciousness. The more directly a verbal idea evokes a determinate object idea, the greater the converse possibility that the object idea itself represent the word in our thought. We are perfectly able to remember our acquaintances without at the same time reproducing their names. Concrete object ideas, like chair, house, table, may also come to consciousness immediately, without the words that denote them. But abstract ideas call be thought only by help of the corresponding words; and of these, again, the particles that are oftenest used naturally have the advantage. We are thus able, without difficulty, to explain the phenomena of progressive amnesia, in functional regard, from the psychological associations, on the one side, and the general effects of functional practice, on the other. Now these phenomena too have, of course, their physiological substrate. Only, the substrate cannot be conceived as in any way stable, given with fixed centres and their connexions, but must be thought of as labile, developing by function itself and continually changing as function changes.
We are led to this conclusion yet more directly by the second of our two groups of phenomena: the symptoms of auxiliary and vicarious function that regularly follow in the train of speech disturbances, and that point again to the universal validity of associations and their gradual establishment by practice. What is called amnestic aphasia still furnishes the most striking illustrations; it shows the phenomena of associative compensation in extraordinary variety. Thus, we not infrequently find that the word for an object is not directly at the patient's disposal, but that it is immediately remembered if other verbal ideas, that often occur in connexion with it, are intentionally reproduced. In one case, which has become classical, the patient, who suffered from an almost total amnesia induced by an injury to the head, was always able to discover the forgotten words by writing them down. Further, if he were required to name the attributes of an object, he failed to do so, both when the object itself was named and when the attributes in question were exhibited to him in other objects, but succeeded, after some improvement in his condition had set in, if he saw the object before [p. 310] him. Cases of this kind, which cannot be included in any rigid schema of localisation, are referred by certain authorities to a 'functional aphasia,' and thus distinguished from the typical forms of cortical and subcortical aphasia. In reality, however, instances of associative compensation are extremely common. We may unhesitatingly assume that if the phenomena are not recorded, and more especially if they are not recorded with minor degrees of derangement, this is merely because they have not been observed or not explicitly verified. All the aphasic disturbances are, indeed, at once functional and anatomical: and the functional disorder does not exclude structural defect as such, but only that rigid localisation which is presupposed in the schematisations of the centres and their connexions. The error that has crept in is the error of inversion. The anatomical conditions have been first laid down, and the functional symptoms that came under observation have then been assigned, as accurately as might be, to determinate cortical areas. But the first requirement evidently is that we analyse the functions themselves, and only then turn to consider how, in the light of this analysis, the anatomical conditions are to be envisaged.
If we take our stand upon this principle, we must necessarily begin by examining these psychological facts front which the associative relations may be ascertained that hold between the individual constituents of a verbal idea or between the different word ideas themselves. For the sake of brevity, these associative relations may he termed, in their entirety, the psychological structure of verbal ideas. We may, further, limit our enquiry to the constitution of consciousness that characterises the adult members of a civilised community, where the artificial development of the capacities of reading and writing has been superinduced by practice upon the natural function of speech. Proceeding on this basis to our task of a general appreciation of these activities, and of the disturbances which occur amongst them, we note, first of all, that every complete verbal idea is a complicative association of three constituents: the phonetic utterance, L; the script form, S; and the meaning contents, B. Each of these is, in turn, made up of two, more intimately compounded elements, Thus, the phonetic utterance, L, is composed of auditory idea (a) and articulatory sensation (m). The script form S is composed of visual idea (o) and movement sensations of writing (m'): these latter, we may suppose, corresponding originally to the pantomimic movements with which primitive man accompanies his talk, and then translated by civilisation later on into the specific form of writing [p. 311] movements. Lastly, the meaning contents B may be analysed, in general, into an ideational component (v), and a feeling (g) dependent both upon this and upon the whole configuration of consciousness, but more especially upon its relations to other contents, past and present. Not only B, however, but the other constituents of the verbal idea, L and S, as well, are associatively connected in the most various ways with further meaning contents and word ideas; so that a given combination L S B never really occurs by itself alone, but always as a formation only relatively isolable from a more or less complicated tissue of associations. Now all the phenomena of speech, and all its disturbances also, indicate that these associations with other verbal and ideational contents vary within wide limits from case to case. But they indicate, further, that the different constituents of one and the same verbal idea may be associated in very different degrees of intimacy: this altogether apart from the fact that the vivacity and activity of these constituents must vary very considerably with the constitution of the individual consciousness with the stage of practice attained, and with the specific conditions under which the ideas are formed. Thus, there are individuals who are but obscurely aware, in ordinary speech, of the elements o, or m, m', of the complex L (a m) S (o m') B (v g); the auditory impression of the word dominates in consciousness. There are others, for whom m is especially prominent; and yet others, for whom even o is constantly noticeable. In reading, o naturally stands in the foreground of consciousness. It is, however, so closely associated with the terms a m that these may always be perceived, more or less clearly, along with it. We find the most complete representation of all three constituents in the process of writing to dictation, where a directly arouses m, m' and o. In this instance, just because the word constituents proper force themselves all together upon consciousness, the meaning contents v g may very easily be relegated to entire obscurity; it is a common experience that writing to dictation slips more readily than any other form of speech function, more readily even than repeating from dictation, into a meaningless routine. The degree of intimacy of the association between the meaning elements v g is further conditioned, as we remarked above when discussing the phenomena of partial amnesia, upon the logical and grammatical value of the verbal ideas. It is owing to the influence of this factor that, in abstract word forms, the ideational constituent v entirely disappears behind the word elements proper, a and o, and only the affective element g remains. We shall treat later of this 'conceptual feeling'; here we have simply to note that it is firmly associated to the word constituents L S.
Finally, there are two further phenomena, connected with this variation [p. 312] in the intimacy of associative combinations, that require a special mention. They bring out, in a striking way, the quite extraordinary variety of tendency and disposition exhibited by the speech functions in the individual case. The first is this: that the closeness of any particular association, of the group here under discussion, depends not only upon the connected elements themselves, but also upon the direction of their connexion. It is but rarely that a speech association is of approximately equal strength on both sides. The principal instance of the kind is the connexion between the two components of the first term L of the complete verbal idea; here, under normal conditions, the auditory impression a is as powerful in arousing the tendency to articulatory movement m as the movement is to evoke the auditory idea. The same sort of reciprocal influence appears to be exerted, though on the whole with somewhat less of constancy, by the auditory word idea a and the feeling g which attaches to the meaning idea. A heard word first of all arouses a feeling of its meaning, before the meaning itself has come clearly to consciousness. This order of events obtains more especially in the case of unfamiliar or entirely unknown words; oftentimes, indeed, the whole process stops short with the arousal of the conceptual feeling. But we also observe, conversely, that in cases where the word that expresses a particular idea is for the moment beyond reach, in cases i.e. where we are "trying to think of" a word, a strong associative influence is exercised by the feeling accompanying the idea; so that it is clearly the association g a, and not (or, at any rate, only in a much slighter degree) the association v a, that is primarily responsible for the success of the act of recollection. In direct antithesis to these associations, in which the strength of the connexion is approximately the same for both sides, are those in which the one direction has a decisive advantage over the other. Here belongs, e. g., the association between o and m, where the direction o m represents a much stronger associative tendency than the direction m o, -- the script form arouses the movement of articulation, but this has very little power to call up the script form; or the association between m and m', where the movements of writing easily arouse the articulatory sensations of the organs of speech, but these require special conditions if they are to touch off the movements; and so on.
As a rule, then, the speech association runs more smoothly in the one direction than in the other. The second complicating condition is this. There is a continual fluctuation, not only in the strength of the connexions at large, but also and more particularly in the preponderance of the one or the other direction under the influence of practice. The effects of practice are seen most clearly in cases where some change has been produced in the normal intensity of the individual constituents of the idea, or where certain associations have lost their efficacy. In deaf-mutism, e.g., the elements am [p. 313] are entirely wanting; This loss is compensated by the development of the elements o m' and of the two-way association between them: m', at the same time, appears in its original character as component of a mimetic movement. If the patient learns to speak, the connexion o m develops as a two-way association of the most pronounced type, and completely replaces the normal connexion a m; and so on.
We may now attempt, in the light of the preceding paragraphs, to construct a plan of the whole number of associations that obtain between the elements of the complete word idea. Let the elements be denoted by the symbols, a, m, o, m', v, g; the associations between them by connecting lines; the direction of the associations by arrows; and their relative stability by the thickness of the lines. The ordinary course of the associations, within a complete word idea, will then be represented, in general
We began this discussion with a review of the 'speech centres.' From the point of view to which we have now attained by our functional analysis of verbal ideas, these centres, mapped out in Fig. 102 in accordance with pathological observations of the lesions underlying disturbances of speech, naturally acquire a very different significance from that assigned them in the plan of localisation and conduction laid down in Fig. 103. For it is clear that the constituents a, m, o, m' of a word idea can be brought into settled relation with definite brain areas only on condition that the associations between them may be regarded as relatively stable connexions. There [p. 313] are, however, two reasons why they may not be so regarded. In the first place, both the intimacy and the direction of the associations are throughout dependent upon individual practice. In the second place, the formation of at least one set of associations, those that include the terms o and m', cannot be explained save as the outcome of a development which is conditioned upon a certain level of civilisation, and which therefore began late in the history of the race. We might accordingly expect to find, as in fact we do, that these associations are subject to especially large individual variations. But variability in the conduction paths necessarily brings with it a certain degree of variability in the centres themselves: a conclusion which we need not hesitate to accept, since it is borne out, from another side, by the phenomena of restitution of function. In fine, then, the expression 'speech centre' cannot, under any circumstances, designate a central organ, in the sense in which this term is ordinarily employed, -- an organ that presides, exclusively or even preponderantly, over a determinate group of functions. It can mean nothing more than an area which contains the most important nodal points of those conductions whose co-operation is indispensable for functions of this particular kind. In other words, the significance of the centres is rather that they comprise the points of connexion than that they include the points of departure of the elementary processes concerned in a certain complex activity. Such a conception harmonises very well with the idea suggested by the effects of practice and of restoration of function: the idea that it is not the centres but, in a certain sense, the functions that are the original things. The functions make their centres, and are constantly modifying them in accordance with the variable conditions of function itself. Hence the localisation of the functions is not stable, but labile; the boundaries of the functional areas are not fixed, but changing, -- subject to the functional influences that modify the conditions of conduction, and with them the actual conduction paths. So it comes about that functional analysis of the very phenomena which first inspired the modern doctrine of strict localisations, the phenomena of derangement of speech, has at every stage thrown the intrinsic impossibility of this doctrine into clear relief. Such a result, however, is, after all, but the natural consequence of the extreme complexity of the speech functions, and of that many-sidedness and variability of the psychophysical conditions which they are peculiarly fitted to bring into open view. Understood in this way, it is also a result that must be generalised. What holds of the speech centres holds, in reality, of the 'sensory centres' as well, despite the fact that, even to-day, they are ordinarily interpreted as simple central projection surfaces. They, too, are 'association centres,' in the sense that they contain nodal points which serve to centralise the functions, but centralise them by bringing into connexion all the different partial functions, sensations, movements, reflexes, synergies of sensations [p. 315] and movements, that work together in the functional whole, -- the connexions still admitting of continual adaptation to external conditions.
(c) -- The Apperception Centre
There is an extensive region of the human brain that appears, so far as sensory and motor symptoms are concerned, to be comparatively indifferent whether to external stimulation or to internal change: the portion of the frontal lobes that lies anteriorly to the anterior margin of the motor zone (Fig. 88, p. 205). Pathological observations show that injuries to this region, sometimes involving the loss of considerable masses of the brain substance. have failed to produce any derangement whatsoever of the motor and sensory functions. As a rule, however, the observers report, with equal definiteness, a permanent disturbance of the mental attributes and faculties. In a famous American case, e.g., a pointed iron rod, one and a half inches in diameter, was driven through the head by the explosion of a blast, entering at the angle of the left lower jaw and emerging near the anterior extremity of the sagittal suture. The patient, who lived twelve and a half years after the accident, gave no indication of disturbance of sensation and voluntary movement, but suffered a complete change of character and activities. "He combines the animal passions of a man with the intellectual activities of a child," so writes the attending physician. In other cases, decay of memory, inability to concentrate the attention, entire loss of will-power, etc., are quoted as characteristic symptoms. These results agree with the observation that the pathological degenerations of brain tissue which accompany the decay of intelligence and will in cases of paralytic dementia usually have their seat in the frontal lobes, and with the general law that intellectual development keeps even pace throughout the animal kingdom with the development of the prosencephalon. It is also said that highly developed human brains are often characterised by an especially abundant formation of secondary gyres and fissures in the frontal lobes. It can hardly be maintained, however, that in these cases there is any marked difference between the frontal and the other, e.g.. the parietal and occipital regions.[p. 316]
On the ground of these facts, it has been suggested by various investigators, among others by MEYNERT, HITZIG, FERRIER and FLECHSIG, that the frontal brain stands in intimate relation to the functions of the 'intelligence.' Now 'intelligence' is an exceedingly complex and indefinite term. If, as we have seen, the act of vision and the different functions concerned in speech are connected with 'centres' only in a very limited sense, it is, of course, impossible to conceive of a localisation of the intellectual functions, or to connect them with a specific 'organ of intelligence.' The utmost that we can say is that that this particular region of the cortex may contain certain nodal points of conductions, whose abrogation produces disturbances of an intrinsically elementary character, but manifesting themselves, in the complexity of functional co-operation, as impairment of the 'intelligence' and derangement of the compound feelings. On the anatomical side, the abundant connexions mediated by association fibres between the frontal brain and other brain regions, furnish a distinct support to the view that the frontal lobes contain nodal points of especial importance for the interrelation of the central functional areas. And the objection that lesions of the frontal brain have occasionally been observed to pass without permanent moral or intellectual injury is not decisive: for local lesions in general are the more easily compensated, by vicarious function of other parts, the more numerous and varied are the connexions of the elements; and this condition is, on the whole, more adequately met by the cortical areas occupied by the 'association centres' than it is by the direct sensory and motor centres. Hence these negative results, in cases of local injury, are far outweighed by the fact, as stated by the brain pathologists, that "lesions of any extent at all are never observed to occur in this region without causing the most serious intellectual defects."
Let us now attempt, so far as may be possible, to analyse the complex phenomena, grouped together under the indefinite rubric of 'intelligence,' into their elementary processes. These processes must be such as can be connected with a clear and simple psychological idea; and this must, in its turn, be capable of correlation with a correspondingly simple physiological idea. We find what we require in the elementary idea of the apperception of a mental contents, e.g. of a sensation. What apperception means in detail we shall show later on: here we understand by it a psychological process in which, on the objective side, a certain contents becomes clear in consciousness and, on the subjective, certain feelings arise which, as referred to any given contents, we ordinarily term the state of 'attention.' Now the objective component of this complex process, the 'clarification'[p. 317] of a contents, is surely suggestive in the highest degree of determinate physiological concomitants. Just as, e.g., when a sensation grows stronger or weaker, there is a parallel increase or decrease of the physiological processes of excitation in particular nervous elements, so must we suppose that, when sensations or other conscious contents grow, as we have put it, clearer or more obscure, these changes are conditioned upon some sort of physiological substrate. And it is evident that this substrate may very well consist of certain simple processes, consistent with the general principles of nerve mechanics; whereas the idea of 'intelligence' is altogether so complicated that the search for any kind of definite or limited physical substrate would, in its case, be entirely hopeless.
It might, at first thought, be supposed that the elementary process of apperception which appears in its simplest form when a sensation becomes clearer, consists, on the physiological side, merely in an increase of the nervous excitation that runs parallel with the sensation; and that the physiological change, when the sensation becomes more obscure, is a corresponding decrease of the same concomitant excitation. But to say that a sensation 'grows clearer' or 'grows more obscure,' is, in reality, a very different thing from saying that it 'increases' or 'decreases in intensity.' To speak relatively, a weak sensation may be clearly, and an intensive sensation obscurely apperceived. And a little introspection suffices to show that a sensation, in growing stronger or weaker, alters its own intrinsic character; while, if it grows clearer or more obscure, the change is primarily a change in its relation to other conscious contents. A particular impression is always apprehended as 'clearer' in contrast to other impressions which, as compared with it, appear obscure. These facts suggest that the substrate of the simple apperception process may be sought in inhibitory processes which, by the very fact that they arrest other concomitant excitations, secure an advantage for the particular excitations not inhibited. If we postulate an inhibitory process of this kind, we are able to explain how it is that apperception as such does not consist in an intensification of the sensation contents. And if we assume, further, that the inhibitory influence, in this special case, is not exerted directly upon certain excitations in progress within the sensory centres, but rather upon the conduction of the elicitations to the higher centres in which the sensory contents are combined to form complex resultants, we avoid doing violence to the obverse fact that the conscious contents obscured by inhibition do not on that account lose in intensity. The arousal of the inhibition, since on the psychological side it is ordinarily dependent upon particular conscious contents, past and present, must be physiologically conceived as analogous to that of the reflex inhibitions occurring in various forms in the lower nerve centres. There is, however, a difference. The inhibitory effects are liberated, here [p. 318] as elsewhere, by certain excitations that are conducted to the centre; but their liberation is at the same time influenced by that incalculable manifold of conditions which, for the most part, we can merely group together under the indefinite name of the current disposition of consciousness, as determined by past experience and the circumstances of the time.
We thus regard apperception as the one elementary process indispensable to any sort of 'manifestation of intelligence' and, indeed, to the higher mental functions at large. The considerations put forward above with respect to its physiological substrate are, of course, hypothetical. We have fewer data in this case even than we had for our discussions of the functions of the visual and speech centres, and we are accordingly thrown back upon conjecture and tentative hypothesis. These must be based, almost exclusively, upon the results of a psychological analysis of the functions. Except for the meagre analogy of reflex inhibition, physiology furnishes us with nothing more than the general principles of nerve mechanics. Nevertheless, it is worth while to attempt a schematic representation of our theory; we can at least show the general possibility of a physiological interpretation of the complex phenomena in question. Such a representation is given in Fig. 105. We assume that the central area of apperception AC stands in connexion with a twofold system of conduction paths: the one centripetal, ss', hh', that brings up sensory excitations from the primary sensory centres; and the other centrifugal, la, gf, etc., that carries, conversely, to subordinate centres the inhibitory impulses proceeding from AC. We then have, according as these impulses are transmitted to sensory or motor centres, either the apperception of sensations or the excitation of voluntary movements. In the former case, it is other sensations, in the latter other motor impulses, aroused by internal or external stimuli, that are forced into the background. It is plain that the transmission here assumed presents a certain analogy to the reflex process, and particularly to the reflex inhibitions. At the same time, the way in which apperception depends upon the sensory excitations coming in at the moment marks a wide divergence from the schema of the reflex mechanism. In the reflex, we find the motor excitation or inhibition following of necessity from the action of the sensory stimuli; in apperception and voluntary movement, we can speak simply of a [p. 319] regulative influence of the excitations in progress, -- the implication being that a large number of intermediate terms, which our methods cannot reach, exert a determining influence upon the final result. The physiological character of these intermediaries is wholly unknown to us. We can only infer, from psychological experience, that definite dispositions take shape in every brain, as the consequence of generic and individual development, and determine the excitatory processes that run parallel with the act of apperception. If, then, we refer the apperceptive acts to a particular physiological substrate, we can do so only on the condition that we endow the central area in question with connexions to the other central parts, in virtue of which the excitations released in it are dependent upon these dispositions. Hence the centrifugal paths that issue from the apperception organ AC and serve to conduct the inhibitory excitations must, in general, take two directions: a centrifugal sensory and a centrifugal motor. In both directions they are connected, both directly and indirectly, by way of intermediate centres that represent nodal points of conduction for certain complex functions, with the direct sensory centres (SC, HC) and the motor centres (MC). The part of intermediary is played, within the centrifugal sensory path, by certain intermediate sensory centres (O and A); within the motor path, by analogous motor centres of complex character (B and L). The term 'centre' is here used, of course, only in the relative sense defined above, in the discussion of the visual and speech centres. We found, e. g., that the speech centres were not to be regarded as independent sources of the functions ordinarily ascribed to them, but simply as indispensable intermediaries in the mechanism of speech associations and apperceptions; and the same conclusion holds here. The physiological significance of these centres may be roughly illustrated in this way. We will suppose that various sensations, belonging to the domain of speech, have arisen in the sense centres proper, SC and HC. The corresponding excitations are at once combined, in the intermediate sensory centres O and A, into an unitary excitation process; whereupon the apperceptive inhibition can operate to render this, or the primary excitations in progress in the centres SG and HC, clearer or more obscure. The processes in O and A will thus have the significance of resultants, which correspond to the functional unification of the two associatively connected elements, phonetic utterance and script form. These resultants must not, we need hardly repeat, be regarded as traces, stamped indelibly upon certain cells, but rather as transitory processes of extremely complex character, embracing a large variety of elements, -- processes akin to the stimulation processes in the peripheral sense organs, and to other processes in the central nerve substance, all of which leave a disposition to their renewal behind them. A like function must be assigned to the intermediate motor centres B and P in which an act of apperception releases (by the paths g f r s, g fr s) a determinate motor excitation, corresponding to the sensory excitations brought up from SC and HC (by ss', hh'), or from O and A (by ek, ek); or else an unmediated activity on the part of the two elements, phonetic utterance and script form, releases (by the paths e f, ef) the corresponding motor impulses, without interference from the apperception centre, i.e. by way of a direct reflex excitation. These impulses are then, in all cases, carried (by the paths f r s, fr s) to the general motor centres MC, whence they are transmitted along the further nerve conduction to the muscles.
In the hypothetical schema of Fig. 105, the paths that lead towards AC between subordinate centres are represented by [p. 320] uninterrupted, the centrifugal paths that lead away from AC by interrupted lines, and the direction of conduction is further indicated by arrows. Besides the direct motor centres MC, and besides the auditory and visual centres SC, HC, chosen as the chief representatives of the sensory centres, the schema includes, as examples of more complex central areas, the four 'speech centres' mapped out in Fig. 102. Suppose, now, that a series of impressions is carried by the optic nerve S to the visual centre SC: we have, taking only the principal cases, the following possibilities. (1) The impressions are not conducted farther. Then the sensations remain in the state of mere perception or indistinct apprehension. (2) An individual impression a is apperceptively enhanced by inhibition of the impressions b c d: the inhibition is released by way of ss' and hh', and conducted to the centre SC by the path la. We then have perception of b c d and apperception of a. (3) Besides the apperception of the impression a, there is a conduction by way of O to the centre A. Here a resultant is released, which produces in the auditory centre HC (by way of ee a) the verbal idea a corresponding to the visual image a. At the same time, by means of inhibitions released in the centres A and SC along the paths ke and ga, the resulting word idea and the phonetic utterance are apperceived. (4) The processes described under (3) combine with (a) a conduction of the resultants from A by way of L to MC (along ef and frs): involuntarily pronunciation of the word designating an apperceived idea; (b) a conduction from AC by way of L to MC (along gf and frs): intentional pronunciation of the word in question; (c) a conduction from HC by way of A to O, and from O again by way of SC to certain other elements, not shown in the Figure: involuntary association of the word idea to the script form. (5) If the original impression a is the script form of a word, we have the following possibilities: (a) direct apperception, again, by means of an inhibition la; (b) conduction from SC to O, and apperceptive inhibitions along the paths la and ke: apperception of a word whose meaning is familiar; (c) conduction from SC to O, and from O by way of A to HC, with the fourfold apperceptive inhibition la, ke, ke and ga: apperception of a visual and of the corresponding auditory word idea (the ordinary process in reading); and so on. For the rest, this Figure, too, necessarily leaves out of account all those moments that, in the nature of the case, cannot be given a place in any pure conduction schema: so, more particularly, the intimacy and direction of the associations, specially indicated in Fig. 104 for the functions of speech; the influences of practice and of vicarious function, which are constantly at work to change the face of the phenomena; and finally the influences, wholly rebellious, like the last, to any attempt at schematic representation, which are exerted, psychologically by the configuration of consciousness, physiologically by the general state of the nervous dispositions, upon the associations and apperceptions in progress at any given time.
§ 8. General Principles of the Central Functions
(a) -- The Principle of Connexion of Elements
THE principle of the connexion of elements may be understood in an anatomical, a physiological, and a psychological sense. It may therefore be formulated in three different ways, each one of them individual, in [p. 321] contents and meaning, but each one again, in all probability, closely related to the others.
Anatomically regarded, the nervous system is an unitary complex of numerous elements; and every one of these morphological elements stands in more or less close connexion with others. This fact of interrelation is expressed in the very structure of the essential elements, the nerve cells. Not only are the connexions mediated, in general, by the cell processes, but the character of these processes, as dendrites and neurite, oftentimes indicates the direction in which the proximate connexions are made. It is the merit of the neurone theory to have shown how this principle of the connexion of elements is exhibited in the morphological relations of the central nervous system. And the merit would remain, even if the theory, in its present form, should ultimately prove untenable.
Physiologically, the principle of the connexion of elements implies that every physiological activity, which is open to our observation and analysis, is composed of a large number of elementary functions, the nature of which we may, under favourable circumstances, be able to infer, but which we can never completely isolate from the given complex activity. In particular, e.g., the physiological process underlying however simple a sensation or muscular contraction is a complex process, involving the activities of many elementary parts. This may be seen at once from its physiological analysis in a given case, whether the analysis be applied directly to the group of actual functions, or whether it be performed inferentially by a study of the connexions of the elements concerned in them. Our discussions of the act of vision and of the functions of speech hear witness, in both directions, to this physiological significance of the principle of elementary connexions.
Lastly, there is a psychological, as well as an anatomical and physiological, formulation of the principle. It means, psychologically, that the simplest psychical contents discoverable by analysis of the facts of consciousness always presuppose, as their physiological substrate, complex nerve processes, the result of the co-operation of many elementary parts. This complexity of the physical conditions of elementary psychical facts manifests itself in two ways: first, in psychological observation itself, in so far as the psychical elements, simple sensations or simple feelings, are always products of psychological abstraction, and never actually occur except in connexions (a simple colour sensation, e.g., is given as a coloured object in space, and so on: and secondly, in the physiological fact that no psychical process can be imagined, however simple it may be, which does not require for its origination a large number of functionally connected elementary parts. Thus, in the arousal of a sensation of light or tone we have not only the action of stimulus upon the peripheral structures, but also and invariably the processes of nervous conduction, the excitations of central elements in the mesencephalic [p. 322] region, and finally certain processes in the cortical centres. If the sensory excitation is of central origin, as is sometimes the case, -- e.g. in memory images, -- then, conversely, it is first co-ordinate centres and then peripheral regions that are involved in it. Hence every conscious contents, though it be as in these instances quite simple, conceived of in isolation from its connexions, and therefore, psychologically, insusceptible of further analysis, is always, physiologically considered, a complicated formation made up of various nerve processes spread over a large number of elementary parts.
(b) -- The Principle of Original Indifference of Functions
The principle of the connexion of elements, in its anatomical and physiological signification, cannot but suggest the hypothesis that, wherever the physiological functions of the central elements have acquired a specific colouring, -- recognisable psychologically, say, in the peculiar quality of a sensation, or physiologically in the release of a muscular contraction or the origination of a secretory or other chemical process, -- this specific character of their activity is conditioned not upon the elements themselves, but upon their connexions. The connexions to which we must thus ascribe a determining influence upon the development of specific functions are, however, not so much the connexions of the nervous elements with one another, as rather their connexions, first, with the organs and tissue elements that directly subserve the functions themselves, and secondly, with the external stimuli by which the organs and elements are adequately excited. As regards sensation, in particular, which in virtue of its psychological significance is the most important of all specific functions, the determining factors cannot be found in the specific energy of any set of nerve fibres or nerve cells, but only in the physical action of the stimuli upon the sense organs and its immediate consequences, viz., certain changes in the sensory elements that serve to transmit the stimulation to the sensory nerves. It is a matter of indifference, for the present argument, whether these elements are themselves nervous in character, as they are in the olfactory mucous membrane and perhaps in the retina of the eye, or whether they simply represent epithelial appendages to the nervous system, as they do in the organs of touch and audition. But if, now, the specific activity of the nervous elements that belong to a particular sense department is the result of development, if it has been acquired under pressure of the external conditions of life, then the hypothesis of an original indifference of function follows of itself. And this principle, once formulated, immediately suggests the further hypothesis that the functional indifference will have persisted in all cases in which special conditions have not been at work to produce specific differences. There are, as a matter of fact, two phenomena which make it extremely probable that such functional indifference has persisted in a high degree [p. 323] among the central elements. The first is this: that a fairly long continuance of the function of the peripheral organs is required, if the sensations of the corresponding sense department are to appear in consciousness. Those who are born blind or deaf, and even those who have lost the sense of sight or hearing in early childhood, lack the sensation qualities of light and sound. And these sensation qualities are evidently wanting at a time when the atrophic degeneration of the central sensory elements, which results where the functions have been abrogated for a considerable period, cannot yet have set in. The second phenomenon is this: that the functional disturbances occasioned by central lesions may be compensated without disappearance of the lesions themselves, i.e. under conditions that force us to assume a vicarious functioning on the part of other elements. But this clearly presupposes that, under stress of the conditions of life: the elements may take on novel functions of a special kind. In such cases, we are able to trace the rise of specific functions during the lifetime of the individual. We are, of course, bound to suppose that the conditions of the principal functional differentiations have been operative during the evolution of the race. Still, the facts just cited prove that we inherit nothing more definite than certain dispositions, which are given with the connexions of the nervous elements; and that the development of specific functions demands the actual discharge of these functions, and is therefore altogether dependent upon the direct action of the vital stimuli during the course of the individual life. This dependence of the elementary nervous processes upon external impressions must now be localised, primarily, in the nervous elements that come into closest contact with the sensory stimuli, i.e. in those situated at the periphery; it may be looked for far less probably in the more central regions, where, as we have said, substitution and exchange of functions play a leading part. Since, therefore, the immediate contents of consciousness always find expression in the elementary qualities originating in direct connexion with the peripheral functions, everything favours the view that the activities of the higher central elements consist solely in the effects which they produce by the combination and, under certain circumstances, by the inhibition of the excitations conveyed to them.
It follows, then, that the principle of the indifference of elementary functions admits, like its predecessor, of an anatomical, a physiological, and a psychological derivation. Anatomically, it is supported by the essential identity of structure that we find throughout the elements of the nervous system. The neurones differ both in form and in extent. But striking as these variations may sometimes be, the structural differences that they exhibit are, at most, such as indicate merely a difference in direction of conduction; and even these are apparently confined to the more highly dif-[p. 324]ferentiated nerve cells (pp. 42 f.). Physiologically, the principle of indifference of function is attested by the uniform character of the forces that reside in the nervous elements. The two mutually supplementary forms of energy that we designated, from their mechanical effects, excitation and inhibition, or positive and negative molecular work (pp. 60, 80), appear throughout as the simple substrate of nervous function. It is true that, in working out the fundamental ideas of a physiological mechanics of the nervous system, we allowed ourselves to be guided, in the first instance, by the phenomena of muscle, i.e. of the mechanical structures appended to that system. At the same time, these peripheral phenomena came into consideration only in their symptomatic significance. We expressed the facts in a certain way, at the instance of external conditions. But, mode of expression apart, we have every reason to assume the essential identity of the nervous processes. Lastly, the principle derives its principal support, on the psychological side, from the fact that the specific differences in the sensory contents of consciousness, if they are of an elementary nature, may always be resolved into qualities of sensation and feeling that depend upon the functions of peripheral elements. In so far, therefore, as the central nervous system is concerned in the higher psychical processes, it must be occupied, not with the origination of new specific qualities, but solely with the indefinitely complex interrelation of these sensory elements of our mental life.
(c) -- The Principle of Practice and Adaptation
'Practice,' in the ordinary acceptation of the term, consists in the perfecting of a function by its repeated performance. Hence the principle of practice, as applied to the functions of the nervous system, signifies that every central element, whether considered by itself or regarded as co-operating in some especial way, determined by the conditions of life, with other like elements, becomes better and better fitted to discharge or to share in the discharge of a particular function, the more frequently it has been called to its service by pressure of external conditions. We are already familiar with the elementary phenomenon of practice, in the increase of excitability by stimulation (p. 75). This elementary phenomenon is common to all elements of the nervous system: it may be demonstrated even in the isolated nerve, though it is observed at its best, and its after-effects are: more persistent, in the connected neurones and neurone chains. We thus have every reason to look upon it as responsible for the marked changes that are continually taking place, as the result of function, in the nervous apparatus and their appended organs: changes that, especially if we extend them beyond individual to generic development, represent the organs themselves as, in large measure, the products of their functions.[p. 325] The obverse of the practice processes is seen, on the other hand, in the decrease and ultimate abrogation of functions which, as the result of functional inactivity, is at the same time connected with degeneration and waste of their morphological substrate (p. 53).
The effect of practice need not be limited to the quantitative enhancement of a given function. It may lead up to new combinations of elementary processes, by which the qualitative value of a complex function is altered, and the function itself, in accordance with the general character of practice, moulded into more complete correspondence with existing conditions. Under these circumstances, the process of practice is termed 'adaptation.' Adaptation, that is, can never be anything else than a result of elementary practice processes. At the same time, it is a more complex process, since it consists, by its very nature, in a number of concomitant practice courses, which culminate, definitely and purposively, in a single combined result. This complex character of adaptation makes it possible, further, that increase of practice in a given direction may coincide with decrease of practice in another, or that certain elements may be gaining in practice while other elements, alongside of them, are losing. Change in the conditions of life may thus render certain nerve paths more practicable, and others impassable. And the same shift of function that appears here, in the closure and improvement of conduction paths, may occur over whole regions of nerve cells. Hence the adaptations of chief significance for the nervous functions are those in which newly practised elements take the place of others, whose activity has been suspended under stress, internal or external, of adverse conditions. In view of their great importance for the central processes we may group the resulting phenomena under a special principle of vicarious function.
(d) -- The Principle of Vicarious Function
Whenever it occurs, whether in the central nervous system or in other physiological departments, the phenomenon of vicarious function is simply a special case of practice and adaptation. In the present context, it may be termed a limiting case, in the sense that it extends to functions which the elements involved have never before been directly called upon to discharge, though they must, of course, have carried within them the latent possibility of their new offices. Habituation to the requirements of vicarious function may, as the preceding argument has shown, be brought about in two ways. On the one hand, elements and complexes of elements which, up to a certain time, performed only some part of a composite function, may afterwards, owing to the functional disability of the elements correlated with the remaining phases of the function, take upon them the duties of the whole. On the other hand, a certain group of elements may be compelled, [p. 326] by the incapacity of other elements with which they are in some way spatially co-ordinated, to play a part that is altogether strange to them. We may denominate the first of these cases a substitution by extension of the area of function, and the second a substitution by aquisition of novel functions. The first form is conditioned upon the original functional interdependence of departments of the nervous system that may be widely remote from one another; the second is conditioned, conversely, upon the spatial connexion of elements between which no original functional relation can be demonstrated. This spatial connexion may itself consist either in the immediate proximity of neurones lying upon the same side of the brain, or in the union of distant areas by association fibres. In the latter case, substitution occurs most often between the symmetrically situated cortical areas of the two cerebral hemispheres, and is mediated by the association systems that run from the one half of the brain to the other. This statement applies, e.g., in all probability, to the functional areas of the speech centres, whose development is usually unilateral (see above, p. 238).
The first of these two forms of substitution, that by extension of function, appears in general as a gradual compensation of the disturbances due to the partial impairment of a functional area of some magnitude, by way of enhancement of activity in other areas which, from the first, took their share in the same total function. These compensations may proceed from the higher centres, which, in favourable circumstances, almost entirely annul the disturbances produced by lesions in the lower; or may, contrariwise, be the work of the lower centres, which to a certain extent, though never completely, make good the loss sustained by the cessation of activity in the higher. Instances of compensation of the former kind, resulting from vicarious function on the part of superior centres, are not uncommon; we have them, e.g., in the gradual disappearance of the disturbances in cases of injury to the cerebellum, or of lesion of the diencephalic and mesencephalic region. An example of the second kind is the partial recovery of functions, normally conditioned upon the co-operation of certain cortical centres, by an enhanced compensating activity in the diencephalic and mesencephalic centres, such as may be observed more particularly in decerebrised animals (pp. 259 ff.). For both forms of substitution, the principle of connexion of elements is, besides that of practice and adaptation, of primary importance. None of these compensations would be possible, if the central function were not always divided into a number of partial functions, in each of which there is co-operation of all the factors necessary to the function as a whole; so that the higher central area contains the fundamental constituents required for the activity of the lower, and the lower in its turn those required for the function of the higher. Thus, to all appearances, we have repeated in the visual cortex, only on a [p. 327] higher plane and in more complicated form, the same relations of sensory and motor conditions that characterise the mesencephalic portion of the visual centres (cf. Fig. 78, p. 185). This is the reason that disturbances arising in the latter may be compensated from the visual cortex, and that, conversely, even cortical defects may, to a certain extent, be made good by the mesencephalic centres. At the same time, these compensations are, on the whole, the less likely to occur, in the case of simple functions, the nearer the lesions approach to the periphery; and are the less complete, in the case of complex functions, the higher the functional centre that is the seat of the disturbance. Interruption of the sensory nervous conductions in the myel and in the peripheral nerves is altogether beyond the reach of compensation by vicarious function. And, on the other hand, the disturbances of sense perception and of its associative connexions that are caused by destruction of cortical areas can never be more than imperfectly compensated by habituation of the lower centres.
The case stands very differently with those other forms of vicarious function which have their source in the spatial connexion, whether direct or mediated by association fibres, of the nervous elements. Here, the newly habituated parts apparently acquire wholly new functions. Under these circumstances, it is natural to suppose that the areas concerned are such as, before their assumption of the vicarious function, discharged no duties whatsoever. Indeed, there are many authors who take this assumption for granted, -- connecting it, for the most part, with hypotheses regarding the complex functions of the elements themselves, -- and who accordingly consider the cerebral cortex, in particular, as a functional department that contains within it a large number of reserve elements, intrinsically functionless, and provided simply as substitutes for elements that may have become defective. Now the fringe of teleological ideas that surrounds this notion of 'functional reserves' would alone make us hesitate to accept it. But, that apart, the hypothesis runs counter to the fact that functionless elements always evince a gradual degeneration and decay (Fig. 22, p. 53). Suppose, e.g., that the 'speech centres' of the right hemisphere were entirely functionless: it would then be impossible to understand why they do not become altogether atrophied in the course of a long individual life, still more in that of many generations. This difficulty disappears if, as the principle of the connexion of elements requires, we hold to the opinion that every complex function presupposes an intricate co-operation of central elements and their connecting conductions. But then we cannot fail to see that there is another hypothesis, indefinitely more probable than that which we have just been discussing: the hypothesis, namely, that, here too, the elements destined to vicarious duty have always had a certain, only a comparatively unimportant share [p. 328] in the normal function; and that the substitution consequently consists, again, simply in an enhancement of activities, along lines already familiar to the elements in their normal state. Looked at from this point of view, the appearance of aphasia after destruction of the speech centres of the left side would not imply that the speech functions had their exclusive seat in this left hemisphere, or, as has sometimes been suggested by those who support the hypothesis of word localisation, that only certain subordinate word forms, e.g. the interjections, are localised on the right. It is much more reasonable to suppose that the co-operation of neurone territories, where it is so extraordinarily complicated as it must be in a function of the complexity of speech, involves wide-spread areas in both halves of the brain; though, as a matter of fact, the areas of the left hemisphere are normally the more practised, and their destruction accordingly produces specially obvious defects. The larger practice of the motor organs on the right as compared with those on the left hand side of the body furnishes an admirable parallel to this functional habituation. Genetically, it is, in all probability, related to the unequal development of the speech centres; and factually, it repeats that inequality point by point. For the organs on the right hand side of the body are the more practised, but they are not the sole vehicles of function; and just because they are not, is substitution by new habituations possible.
(e) -- The Principle of Relative Localisation
There can be no doubt that, in a certain sense, the central functions, like those of the peripheral organs, are spatially distinct. But there can also be no doubt that the central organ, as its names implies, represents in contradistinction to the peripheral organs a centralisation and thus, at the same time, an unification of functions; so that any absolute localisation of function, which should confine each separate activity within fixed limits, is a priori impossible, as it is also unsupported by the facts of observation. In the peripheral organs, where the demands of external function have produced diversity of structure, the principle of division of labour is strictly observed, and the localisation of function follows in the train of its observance. In the centres of the nervous system, the principle is broken through in two different ways. On the one hand, every central function divides, and divides the more definitely the higher its place in the ascending series of activities, into a number of subordinate and auxiliary functions, which of themselves embrace wide and, in part, widely remote areas of the central nervous system. On the other, the processes of practice, adaptation and vicarious function show that the spatial centralisation of a function is not fixed, but dependent upon its exercise, and upon the conditions under which this exercise is placed, so that any rigid spatial limitation is out of the ques-[p. 329]tion. The name of 'visual centre,' e.g., is by no means to be restricted to the region of the occipital brain known as the 'visual cortex.' The. nodal points of the optic conduction in the lower nerve centres have good right to share in the title, seeing that they are not only constantly involved in the normal functions of vision, but are also able, to a certain extent, to do substitute duty for the functions of the higher region. Since, therefore, the principle of vicarious function, like the principle of practice and adaptation, implies a dependence upon external and internal conditions which allow the elementary functions a certain freedom of exchange, the localisation of the central functions at large cannot be looked upon as anything more than relative, i.e. dependent in this way upon the functional conditions of the time, and varying with their variation. The principle of localisation thus stands in the closest relation to the principles of the connexion of elements and of the original indifference of function. For without the connexion of elements that is required by every, even the simplest form of central activity, and without an original and, in the case of many central elements, a permanent functional indifference, there could be no shift of the limits of a function with change in its conditions. In fine, then, the principle of relative localisation gathers up and includes all the preceding principles, as its necessary presuppositions; while an absolute localisation of the central functions, such as is oftentimes assumed, comes into direct conflict with every one of them.
The five principles laid down in the text have gained ground but very slowly in the development of the theory of the central functions; and even at the present day they have failed, in many instances, to overcome the opposing views. Their progress was hindered, from the outset, by the authority of scientific tradition; in some measure, more particularly in the domain of anatomical and physiological research, it is so hindered still. Rightly to appreciate this resistance, we must remember that each one of them was obliged, before it could gain acceptance for itself, to oust a diametrically opposite opinion. These five dogmatic preconceptions had practical possession of the field; and the advance of modern nerve physiology has consisted in their gradual refutation, point for point, under compulsion by the facts.
(1) In opposition to the principle of the connexion of elements many physiologists, even now, prefer to posit an autonomy of the elements. Their assumption is, not that all contents of consciousness, even the simplest, presuppose complex functions in which numerous physiological elements take part, but contrariwise that these physiological elements, the nerve cells, can mediate extremely complex psychical functions. Thus, a single nerve cell may, according to circumstances, be the vehicle of a sensation or of a compound idea, a concept. The hypothesis is very seriously intended: its supporters have been at the pains to estimate the number of ideas that, on emergency, may be lodged in an individual consciousness, by counting the number of cells in the cerebral cortex. Some-[p. 330]times, it is true, an attempt is made to mitigate its crudity by the remark that it is merely 'provisional' and 'tentative.' But even in this modified form it lacks all justification. For a provisional hypothesis is of use only so long as it groups the known facts together in a formula that can further the progress of investigation. If, on the other hand, the hypothesis points us to a road that undoubtedly leads in a direction diametrically opposed to the truth, then it has become nothing else than a pernicious prejudice.
(2) In opposition to the principle of indifference of function it is generally held, at the present time, that the law of specific energy, as it is termed, represents an especially valuable asset of modern nerve and sense physiology. Nevertheless, the history of the law shows a gradual regression. It slowly withdrew from the elements where advancing investigation had proved it to be untenable, to other elements whose functional attributes were less thoroughly worked out; and it has finally intrenched itself in those whose real differences of structure and function warrant the assumption of distinct modes of activity. First of all, a specific function was attributed to the nerve fibres. Then, as physiologists gradually became accustomed to consider the nerves as relatively indifferent conductors of the nervous processes, the nerve cells were selected as the vehicles of the specific functions. To-day, when the uniform character of these central elements also is impressing itself more and more strongly upon the investigator, we may confidently predict that the specific function will continue on its travels until it ultimately comes to rest in the peripheral sensory elements. Here it may, with a certain conditional propriety, be allowed to remain; whereas the central elements come into account only indirectly, in so far, i.e., as the influences of practice and adaptation play a part among them.
The law of specific energy still holds its own in current scientific thought. More headway has been made against the dogmas that stand out in contrast and opposition to the principles of practice and adaptation and of vicarious function: the hypotheses (3) of the originality and (4) of the immutability of functional attributes. Neither possesses any present power in nerve physiology, save as it is properly a logical consequence of the strict interpretation of the law of specific energy. The numerous facts that bear witness to the influences of practice, adaptation and vicarious function will not be deprived of their rights. But this merely aggravates the division in modern physiological theory, and shows that it cannot maintain itself for any length of time. The same intrinsic unsoundness appears (5) in the hypothesis of an absolute localisation, which still persists in opposition to the principle of a merely relative localisation. It is the connexion of this hypothesis with those of specific energy and of the more or less complex character of the function of the nervous elements that constitutes the logical foundation of the phrenological edifice. And the constantly recurring tendency to a revival of phrenology in some modified form, adapted to the conceptions of current anatomy, physiology and psychology, is therefore an evidence not only of the extreme vitality of these dogmas, but also of their internal connexion. Indeed, if we grant the specific energy of the elementary functions, we have at the same time admitted their originality, their constancy, their complex character and their absolute localisation. Conversely, all these assumptions fall to the ground, as soon as we recognise the just claims of practice and adaptation. For then we must grant the possibility, within certain limits, [p. 331] of vicarious function; and from this admission follow, in the last resort, a merely relative localisation of functions and, as regards the elements, an original indifference, which by the great majority of the central elements has in all probability been retained as a permanent possession. The five principles discussed above thus form an interdependent whole, no less than the five antagonistic dogmas of the older nerve physiology. That a view of the central functions which accords with the general status of our physiological and psychological knowledge should meet, even to-day, with serious opposition is, perhaps, sufficiently explained, first, by this very interdependence of the traditional doctrines, in which each assumption serves to support the rest; again, by the simple formulation of the older theories, as compared with the larger demands made by the new upon the physiological and psychological analysis of the phenomena; and, lastly, by the natural longevity of ingrained prejudice. To suppose that the specific contents of a sensation is given of itself with the existence of a central element; or that the act of vision is completed with the 'projection' of the retinal image upon a central sensory surface; or that 'memory for words,' 'intelligence,' and what not, that figure in popular psychology as simple and undivided concepts, are localised as simply in sharply circumscribed regions of the brain: to suppose all this is, naturally, very much easier than it is to work out the conclusions that follow from the five principles enunciated in the text. But apart from the objection that these various suppositions run counter to the facts, they are once and for all impossible, for the reason that they rest upon a wholly untenable psychology, upon erroneous physiological ideas, and, in the last instance, upon an antiquated conception of the structure of the nervous system. If, therefore, the anatomists are still to be found to-day among the most zealous champions of the phrenological view, they have really atoned for this fault in advance, by contributing the most valuable materials from which others may construct a tried and tested theory of the central functions.
 See above, pp. 43, 99.
 H. E. HERING and SHERRINGTON, in PFLÜGER'S Arch. f. d. ges. Physiol., lxviii., 1897, 222.
 See above, p. 88.
 See above, Ch. iii., pp. 80, 94, ff.
 MOTT and SHERRINGTON, Proc. of the Royal Soc., lvii., 1895, 481: C. BASTIAN, ibid., lvii., 89.
 SHERRINGTON, in THOMPSON YATES' Labor. Reports, i., 45, 175.
 These and the other mimetic reflexes are of great psychological importance, and are accordingly discussed under the hearting of expressive movements in Part IV., Ch. xvii.
 For the relative autonomy of the cardiac movements, see especially T. W. ENGELMANN. in PFLÜGER'S Arch. f. d. ges. Physiol., lvi., 1894, 149; for the autonomous functions of the movements that fall within the sphere of the spinal reflexes, see GOLTZ and EWALD. ibid.. lxiii., 1897, 362.
 NOTHNAGEL, in VIRCHOW'S Archiv., xliv., 4. BINSWANGER, Arch. f. Psychiatr., xix., 759.
 See above. Ch. i., pp. 29 ff., and below, Chs. xvii., xviii., on Will and Consciousness.
 GOLTZ and FREUSBERG, in PFLÜGER'S Arch. f. d. ges. Physiol., xiii. 1876. 460.
 MASIUS, Bulletin de l'académia de Belge, xxiv., xxv., 1867, 1868.
 HEIDENHAIN, Physiol. Studien, 1856, 9. WUNDT, Lehre von der Muskelbewegung, 1858, 51 f.
 KUSSMAUL and TENNER, in MOLESCHOTT'S Untersuchungen zur Naturlehre des Moonstone, iii., 1857, 77.
 LUCHSINGER, in PFLÜGER'S Arch. f. d. ges. Physiol., xiv., 1877, 383.
 MOSSO, Ueber den Kreislauf des Blutes im menschlichen Gehirn, 1881, 74 ff.
 Cf. with this the discussion of the psychology of dreams. Part V., Ch. xx. The excitatory influence of the dypnoeic blood, mentioned in the text, is confirmed by the fact that other forms of automatic or reflex stimulation -- dyspnoeic spasms, epileptiform twitches, and the like -- are especially apt to occur during sleep.
 WERNICKE, Lehrbuch d. Geirnkrankheiten, ii., 10. KRAEPELIN, Psychiatrie, 6te Aufl., i., 54.
 WOLFF, Allg. Zeitschr. f. Psychiatrie, xxvi., 273. ZIEHEN, Sphygmographische Untersuchungen bei Geisteskranken, 1887.
 For this psychological aspect of mental derangement, and for an account of sleep and of similar states (hypnotism), see Part v., Ch. xx.
 Here belong more particularly the experiments of LONGET (Anatomie et physiologie du système nerveux, 1842; German trans. by HEIN, i., 385); SCHIFF (Lehrbuch d. Physiol., i., 342); VULPIAN (Physiol. du système nerveux, 658), and others. These experiments will always retain a place of honour in the history of the experimental physiology of the central organs. But, from the modern point of view, they must be pronounced antiquated; if only for the reason that they attempt to ascertain the functions of the parts by a purely symptomatic method, from the phenomena of abrogation and stimulation, without regard to morphological relations and the course of the conduction paths.
 FLOURENS, Recherches expér. sur les fonctions du système nerveux, 2nd ed., 1842.
 GOLTZ, Beitrage zur Lehre von den Functionen der Nervencentren des Frosches, 1869.
 CHRISTIANI, Zur Physiologie des Gehirns, 1885.
 GOLTZ, Der Hund ohne Grosshirn, in PFLÜGER'S Arch. f. d. ges. Physiol., li., 1892, 570.
 J. STEINER, Die Functionen des Centralnervensystems, Abth. 1-4, 1885-1900.
 J. STEINER, Die Functionen des Centralnervensystems, ii., 211 ff.
 GOLTZ, Die Functionen der Nervencentren des Frosches, 65.
 SCHRADER, in PFLÜGER'S Arch. f. d. ges. Physiol., xli., 1887, 75.
 SCHRADER, in PFLÜGER'S Arch. f. d. ges. Physiol., xliv., 1888, 175.
 CHRISTIANI, Zur Physiologie des Gehirns, 25.
 GOLTZ, in PFLÜGER'S Arch. f. d. ges. Physiol., li., 570 ff.
 FERRIER, Functions of the Brain, 1886, 412. NOTHNAGEL was unable to discover any marked symptoms whatsoever, even after extensive lesions of the thalami. See VIRCHOW'S Archiv. lviii., 429 and lxii., 203.
 BECHTEREW, in PFLÜGER'S Arch. f. d. ges. Physiol., xxxiii., 413.
 STEINER, Die Functionen des Centralnervensystems, iii., 79 ff.
 SCHIFF, Lehrbuch d. Physiol., ii., 343.
 STEINER, op. cit., ii., 106; iii., 126; iv., 54 ff.
 GOLTZ, in PFLÜGER'S Arch. f. d. ges. Physiol., li., 520 ff.
 NOTHNAGEL, Topische Diagnostik, 204 ff. VON MONAKOW, Gehirnpathologie, 586 ff.
 WERNICKE, Lehrbuch der Gehirnkrankheiten, i., 370.
 NORTHNAGEL, in VIRCHOW'S Archiv, lvii., 209.
 MAGENDE,[sic] Leçons sur les fonctions du système nerveux, i., 280. Cf. also SCHIFF Lehrbuch d. Physiol., i., 340.
 NOTHNAGEL, Topische Diagnostik, 262 ff. VON MONAKOW, Gehirnpathologie, 584.
 FLECHSIG, Plan des menschl. Gehirns, 41.
 LUCIANI, Il cerveletto, nuovi studi, 1891, 49. The results of these experimental investigations, the most detailed made upon the cerebellum, serve in general to confirm the statements of NOTHNAGEL (VIRCHOW'S Archiv, lxviii., 33), FERRIER (Functions Of the Brain, 174), and BECHTEREW (PFLÜGER'S Arch. f. d. ges Physiol., xxxix., 362), as well as the older observations of SCHIFF (Lehrbuch d. Physiol., i., 353). The only point upon which there is some divergence of opinion concerns the direction of the imperative movements set up by unilateral transsections. The differences are probably due to the fact that the cerebellar peduncles were cut at different places.
 LUCIANI, op. cit. 32. LADAME, Hirngeschwülste, 93. WERNICKE, Gehirnkrankheiten, iii., 353. VON MONAKOW, Gehirnpathologie, 624.
 In a case in which the cerebellum and pons were entirely lacking, voluntary movements were possible, but there was pronounced muscular weakness; the patient frequently fell, and her intelligence was extremely defective (LONGET, Anatomie et physiologie du système nerveux, i., 764). Observations by KIRCHHOFF, on certain cases of atrophy and sclerosis of the cerebellum, confirm this account upon all essential points (Archiv f. Psychiatrie, xii., 647 ff). In a case of HITZIG'S, where, it is true, atrophy was only partial, the intelligence was affected, but there was no disturbance of movement. HITZIG himself supposes that the symptoms indicate a large measure of vicarious functioning, especially by parts of the cerebrum (ibid., xv., 266 ff.).
 PURKINJE, in KUST'S Magazin d. Heilkunde, xxiii., 1827, 297. HITZIG, Das Gehirn, 196 ff. Der Schwindel, 1898 (off-printed from NOTHNAGEL'S Pathologie, ix.), 36 ff.
 FLOURENS, LUSSANA and RENZI also observed effusion of blood in the cerebellum as a result of intensive alcoholic poisoning; see RENZI in SCHMIDT'S Jahrbuch, cxxiv., 158.
 See Part III., Ch. xiii.
 BECHTEREW, Die Leitungsbahnen im Gehirn und Rückenmark, 361.
 VON LEYDEN and GOLDSCHEIDER. Die Erkrankungen des Rückenmarks. In NOTHNAGEL'S Handbuch d. Pathologie, x., 149.
 LUSSANA, Journal de la physiol., v., 418; vi. 169. LUSSANA and LEMOIGNE, Fisiologia dei centri nervosi, 1871, ii., 219.
 FLOURENS, Recherches expérimentales, 2nd ed., 28.
 In view of the close connexion of the olives with the conduction paths of the cerebellum (see pp. 170 ff.), it is readily intelligible that injury to these centres should produce motor disturbances akin to those set up by injury to the cerebellum itself. Such disturbances have, as a matter of fact, been observed by BECHTEREW (PFLÜGER'S Arch. f. d. ges. Physiol., xxx., 257). BECHTEREW found, further, that similar disturbances of equilibrium uniformly result from injury to the walls of the diacele (ibid., xxxi., 479).
 LUCIANI, Il cerveletto, Germ. trans., 282.
 FERRIER, in Brain, xvii., 1894, 1 ff.
 Edition of 1874, 220.
 KAHLER and PICK, Beiträge zur Pathol. u. pathol. Anat. d. Centralnervensystems, 1879. 58.
 HITZIG, Der Schwindel, 42 ff.
 Cf. the discussion of the voluntary actions. Part IV., Ch. xvii.
 GALL, Anatomie et physiologie du système nerveux, iii., 1818, 85. COMBE, On the functions of the cerebellum, 1838.
 R. WAGNER, Göttinger Nachrichten, 1860, 32.
 LUSSANA. Journal de la physiologie, v., 140.
 On phrenology in general, see below, § 6.
 LUCIANI, Il cerveletto, Germ. trans., 198. Cf. also FERRIER, Functions of the Brain, 178.
 LADAME, Hirngeschwülste, 99.
 Cf e.g., NOTHNAGEL, Topische Diagnostik, 78 ff. VON MONAKOW, Gehirnpathologie, 635 ff.
 LADAE. Hirngeschwülste, 186 f. NOTHNAGEL, Topische Diagnostik der Gehirnkrankheiten, 435 ff. VON MONAKOW, Gehirnpathologie, 376 ff.
 GOLTZ, in PFLÜGER'S Arch. f. d. ges. Physiol., xlii., 1886, 484.
 LOEB, in PFLÜGER'S Arch. f. d. ges. Physiol., xxxiv., 1884, 67.
 GOLTZ, ibid., lvi., 1899, 411.
 GOLTZ, ibid., xxxiv., 1884, 450; xlii., 1888, 439.
 LONGET, Anatomie et physiologie du système nerveux, Germ. trans. by HEIN, i., 539 ff.
 Cf. the discussions of the centres of vision, speech and apperception, § 7, below.
 See below, Parts IV., V.
 LEURET and GRATIOLET, Anatomie comparée du système nerveux, ii., 290. OWEN, Anatomy of Vertebrates, iii.
 HUSCHKE, Schädel, Hirn und Seele, 60. H. WAGNER, Massbestimmungen der Oberfläche des grossen Gehirns, 1864, 33.
 TIEDEMAN, Das Hirn des Negers mit dem des Europäers und Orang-Utangs verglichen. 1837. BROCA, Mémoires d'anthropologie, 1871, 191.
 GALL and SPURZHEIM, Anatomie et physiologie du système nerveux, ii., 251.
 R. WAGNER, to whom we owe these reproductions, and those of certain other brains of eminent men (Dirichlet, C. Fr. Hermann, etc.), was himself somewhat hesitant about drawing this conclusion (Göttinger gel. Anz., 1860, 65; Vorstudien zu einer wissenshaftl. Morphologie und Physiologie des Gehirns, 1860, 33). C. VOGT, however, justly remarks that it follows, without any question, from WAGNER'S own figures, if we select from them the illustrations that really apply to individuals of acknowledged mental pre-eminence. See also BROCA, Mémoires d'anthropologie, 155. For the rest, it need not be pointed out that, here as elsewhere, the concurrent factors of race, height, age, sex, must be taken into consideration. A normal Hottentot brain in the skull of an European would, as GRATIOLET observed, spell idiocy.
 BOUILAUD, Recherches cliniques propres à demontrer que la perte de la parole correspode à la lésion des lobules antérieures du cerveau et à confirmer l'opinion de M. GALL etc. In Arch. gén. de méd., viii., 1825.
 FLOURENS, Recherches expér. sur les fonctions du système nerveux, 2nd ed. 1842.
 See Ch. vii. below.
 DESCARTES, Les passions de l'ame, I. See above, p. 124.
 See the list in HALLER, Elementa physiologioe, iv., 1762, 397.
 See the historical critique of the doctrine of irritability in the author's Lehre von der Muskelbewegung, 1858, 155.
 Elem. physiol., iv., 395.
 Ibid., 397.
 "Hypothesin esse video et fateor," he cautiously adds. Ibid., 399.
 GALL and SPURZHEIM, Anatomie et physiologie du système nerveux, i., 1810. Cf. also the same authors' Recherches sur le système nerveux, etc., 1809 (contains the memoir presented to the French Institute and the report of the Commissioners). GALL'S two main services to brain anatomy are his introduction of the method of dissection from below upwards, and his demonstration of the universally fibrous character of the alba.
 GALL'S system is set forth in detail in vols. ii.-iv. of the Anatomie et physiologie.
 COMBE, System of Phrenology. 1825 (Germ. trans. by HIRSCHFELD, 1833, 101 f.).
 Anat. et physiol., iv., 341.
 Ibid., i., 271; ii., 372.
 P. J. MÖBIUS, Franz Joseph Gall, in SCHMIDT'S Jahrbuch d. Medicin, cclxii., 1899, 260.
 P. J. MÖBIUS, Ueber die Anlage zur Mathematik: mit 51 Bildnissen, 1900.
 A criticism of the teachings of phrenology from the standpoint of comparative anatomy is to be found in LEURET, Anatomie comparée du système nerveux, i.; a criticism based upon the writer's physiological experiments, in FLOURENS' Examen de la phrenologie, 1842.
 MAGENDIE, Leçons sur les fonctions du système nerveux. 1839.
 FLOURENS, Recherches expér. sur les fonctions du système nerveux, 2nd ed. 1842.
 Cf., e.g., BURDACH, Vom Bau und Leben des Gehirns, iii. ARNOLD, Physiologie, i., 836. HUSCHKE, Schädel, Hirn und Seele, 174.
 BROCA. Sur la siège de la facult du language, 1861.
 Cf. especially his discussions in PFLÜGER'S Arch. f. d. ges. Physiol., xx., 1879, 10 ff.
 Cf. Part II., Ch. viii.
 See below, Part III., Ch. xiv., on visual ideas.
 WERNICKE, Der aphasische Symptomencomplex, 1874.
 LICHTHEIM, Brain, a Journal of Neurology, vii., 1885, 437. A still simpler plan, including merely the motor and sensory centres, was suggested by WERNICKE (op. cit.); it forms the basis of LICHTHEIM'S schema, and is represented in Fig. 103 by the unbroken lines. A plan resembling LICHTHEIM'S, and factually accordant with it, but somewhat more complicated in outward appearance, had been drawn some years earlier by KUSSMAUL: Die Störungen der Sprache, 1877, 183.
 Cf. with this discussion the critical remarks in my Völkerpsychologie, i., 1, 1900, 495 ff.
 S. FREUD, Zur Auffassung der Aphasien, 1891. STÖRRING, Vorlesungen über Psychopathologie, 1900, 127 ff.
 KUSSMAUL, Störungen der Sprache, 163 f.
 GRASHEY'S case, Archiv f. Psychiatrie, xvi., 694 ff. This case, which is one of great interest, has been further investigated by R. SOMMER (Zeits. f. Psych. U. Physiol. d. Sinnesorgane, ii., 143) and G. WOLFF (ibid., xv., 1 ff.). Cf. the detailed discussion in my Völkerpsychologie, i., 1, 1900. 502 ff., and in STÖRRING, Vorlesungen über Psychopathologie, 132 ff. A survey of the very extensive modern literature on aphasia and amnesia is given by O. VOGT, Zeits. f. Hypnotismus, vi., 1897, 215, 266 ff. Cf. also PICK, Arch. f. Psychiatrie, xxviii., 1 ff., and C. BASTIAN, Aphasia and Other Speech Defects, 1898.
 Cf. the fuller psychological discussion of the processes of association in Part V., Ch. xix. below.
 This schema is taken from my Völkerpsychologie, i., 1. 1900, 519 ff. (section on the psychical structure of word ideas). The reader is referred to this passage for further explanation the Figure.
 Cf. the cases collected by CHARCOT and PITRES (Revue mensuelle. Nov., 1877), FERRIER (Localisation of Cerebral Disease, 1878), and DE BOYER (Études cliniques, 40, 54); also BIANCHI, in Brain, xviii., 1895, 497.
 The report is printed by FERRIER, op. cit.
 Cf. DE BOYER, 45, obs. iv.; 55, obs. xxvii.
VON MONAKOW. Gehirnpathologie, 491 ff. The latter author
reports a similar case from the Zurich clinic, and cites other analogous observations of JASTROWITZ.
 MEYNERT, Vierteljahrsschrift f. Psychiatrie, 1867, 166.
 FLATAU and JACOBSOHN, Handbuch d. Anat. u. vergl. Anat. d. Centralnervensystems d. Säugethiere, i., 536 ff. MARCHAND, Die Morphologie des Stirnlappens, in Arbeiten des pathol. Instituts zu Marburg, ii., 1893.
 H. WAGNER, op. cit.
 The reader may refer, for purposes of comparison, to the figures of the brains of GAUSS and of an individual of moderate intelligence, Figg. 100, 101, p. 285, above.
 ZIEHEN, Leitfaden der physiol. Psychologie. 5te Aufl., 195; Introduction to Physiological Psychology, 1895, 231.
 VON MONAKOW, Gehirnpathologie, 492.
 Part IV., Ch. xvii.; Part V., Ch. xviii.
 See p. 53 above; and cf. Part II., ch. viii., below.
 MEYNERT, Vierteljahrsschrift f. Psychiatrie, i., 1867, 80. H. MUNK, Ueber die Functionen der Grosshirnrinde, Einleitung, 9.
 For the limitations to be put upon the law of specific energy in its application to the peripheral sensory elements, cf. below Part III., ch. viii., § 4.