Classics in the History of Psychology

An internet resource developed by
Christopher D. Green
York University, Toronto, Ontario
ISSN 1492-3173

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Introduction to Psychology

George Sidney Brett (1929)
Authorized by the Minister of Education
First published in Toronto: Macmillan of Canada

Posted November 2001


Chapter IV

The Mechanism of Action



The history of the human race has always been a record of two kinds of achievement, the physical and the mental.  It is not necessary to begin any subtle discussions about soul and body or mind and matter: we require to approach the subject with no more or less than the simple practical distinctions of daily life.

Every one knows: what is meant by manual work: and in the same way every one knows what is meant by intellectual work. Ordinary people use words which economize labour.  When an action is called "manual", the word is not used in such a way as to include all thinking: and when work is called "intellectual", the word does not mean that nothing is happening at the same time in any other organ of the body except the brain. This warning against exaggerations would not be necessary if there were not so many people to-day anxious to invent theories called "idealistic" or "materialistic", which, in fact, no reputable thinker ever held.

What the reader should never forget is the necessity of making distinctions and the fact that all distinctions are artificial.  We very rightly say that we think with the brain: but if the brain is not properly nourished, if the blood-stream is not of the right quantity and quality, the brain will not function at all.  In short, the real agent is the organism: manual and intellectual [p. 37] work are different ways in which the organism achieves the total end of action.

Since the organism acts as a whole, it is impossible to understand an action of any kind without a plan of the whole organism. For this reason a psychologist requires to know such anatomy and physiology as will help him to realize how the different parts of a total reaction or response are united. The most important part of the organism for this purpose is called the nervous system: after the nerves the muscles and glands need to be considered. It may be argued that the list should be extended; that all bodily changes should be considered; that the chemistry of digestion, for example, is an important factor.  This argument would be quite valid, and biochemistry does in fact contribute many important facts, but the answer to this argument is the old proverb, "Art is long, life short".  Many things must be taken for granted in order that a few may be selected for special concentration.



The existence of nerves in the body was discovered in the third century before Christ. At a very early date experiment showed that motion and sensation were dependent on different groups of nerves.  But more exact knowledge was not attained until the beginning of the nineteenth century.  The anatomists proceeded from "gross anatomy" (the general position of bones and muscles), to the finer anatomy of tissues (histology) and nerves (neurology). At a later date the study of pre-natal growth (embryology) revealed the stages of development and greatly increased our knowledge of the structure of the brain and nerves. To this were added the results of many experiments which showed the localization of areas on the brain surface differing in structure [p. 38] and function. The differences in structure of the surface layer or cortex are due to the formation of distinct layers of cells at different stages of development.  These facts and the terms now introduced will be explained as we come to them.

It is best to begin by forming a general picture of the organism.  The outer surface (periphery) is the boundary of the body and the area which is exposed to all stimulation from without.  This surface is in various ways sensitive, because in it are the ends of nerves which run from that surface inward to some centre. In the case of animals that are not highly organized, this  centre will be a relatively independent knot (ganglion): but as we ascend the scale of animal life, there is a greater complexity of arrangement and the formation of multiple nerve-centres (ganglia) each controlling a segment of the animal.  By nervous connections between these ganglia the segments of the animal gradually become less and less independent, until, eventually, it become necessary to develop a controlling ganglion at the foremost end of the animal -- the higher nerve-centre or brain.

This evolution of a higher centre for control of lower centres is called the integration of the nervous system. Integration is a very important fact and must be constantly kept in mind.  All the possibilities of co-ordination, that is making many separate movements harmoniously at one time, depend on integration.  The opposite -- disintegration -- is equally important in all cases where we have to explain inability to co-ordinate either movements or ideas.  The body, as we know it, is the product of a very long development, and the stages of its growth are marked by levels of integration.

The first and most essential functions were nutrition and growth; this system was first built up. Then the [p. 39] range of action was increased, and the receptors, such as nose and ear and eye, became specialized organs. Finally, the inner mechanism was so far integrated as to form the rudimentary brain.

Human development, speaking generally, follows this line of development, and the fact is important because it explains at once the necessity of genetic psychology; for, by studying "genesis" or development in time, we learn the right order in which the forms of training should be used so as to meet the requirements of different ages from infancy to maturity.

For convenience in stating the facts we began with the surface and the senses.  But these are not to be separated from the mechanism of motion which is organic response to stimulus.  The organism in all higher animals is sensori-motor.  The instrument of motion is the muscle.  The motor nerves start from the nerve centres in animals and control the muscular response. Muscles, tendons, and joints all contain sense-organs, known as "proprio-ceptors", so that the higher centres are kept informed of changes in those parts, a third class of nerves called "intero-ceptors" are tile means by which we become aware of stimulations of our internal organs, such as the alimentary canal.  A summary of this brief account can be made as follows:

1. The main classes of nerves are sensory and motor.

2. Sensory nerves are classed according to their functions as extero-ceptor, proprio-ceptor, intero-ceptor.

3. The unit of action consists of the sensory (afferent) and the motor (efferent) nerve, with the connector.  'This unit is called a reflex are because it may function in some cases independently (v. p. 31). [p. 40]



In ordinary language the word "nerve" is used in a general sense.  The minute investigations of modern anatomists have shown that a nerve is in reality a structure which has a variety of distinct parts.  As this fact has some relation to the problems of conduct, it is necessary to understand what is known about the structure and function of nerves.


When people think of a nerve, they think of a slender, string-like object, of a whitish colour, and soft to touch. This is really a nerve fibre inclosed in a sheath of fatty substance called the medullary sheath.  If this nerve fibre was traced back from its termination in the skin surface or other locality, it would prove to be a process or outgrowth of a cell-body which consists of a nucleus [p. 41] and a quantity of protoplasm immediately surrounding it.  A nerve is thus seen to be much more complex and organized than it appeared to be. To avoid confusion with older ideas the nerve cell with all its branches is called a "neurone".


A neurone is a physiological unit comprising (1the nucleus, (2) the protoplasm surrounding it, and (3) two branches growing out from the cell body.  One of these branches is a kind of main branch, and is called the "axis-cylinder" or "axon".  The other reaches out a shorter distance and is called a "dendrite" (Greek for "tree") because it looks very much like a tree branch with fine twig-like extensions.  These outgrowths from the [p. 42] body of the nerve cell must ;be distinguished from the side branches which are put out by the axis-cylinder and connect one neurone with another.

The sensory neurones have processes by which they are attached to the posterior part of the spinal cord, and their cells lie outside the cord. The motor nerves, on the contrary, are the axonal processes of cells lying in the anterior part of the cord. These are the essential facts about the structure and position of the sensory and motor nerves.


The nerves act as conductors. The process of conduction is most probably a chemical process.  It consists of a series of changes passed on from cell to cell, somewhat like the passage of a spark along a fuse, and capable of going in one direction only.  It is called the nervous impulse.  The neurones are not actually continuous according to the usually accepted view, though [p. 43] there is still some dispute about these difficult points of microscopic analysis.  Whether continuous or not, the neurones have some form of conjunction which is called "synapse" (Greek synapto, join together).  It is a form of connection between the end of an axon of one neurone and the end of a dendrite of another neurone. The place of the synaptic union offers some special degree of resistance to the transmission of the impulse along the neurones.

This point in the theory of neural action is important, because it must be assumed that the state of affairs at a synapse will determine the total course of the impulse and the ease or difficulty with which the wave of impulse continues its course. We may assume that the facility of a response is explained by the reduction of the resistance at the synapses, which would be the equivalent of that facilitation which results from habit, that is to say the repeated use of a certain track through certain synapses.  In the higher centres it might [p. 44] explain the capacity for acts of memory, which would be a positive association between brain areas through which the stimulus passes.  Since habit is a, kind of memory and memory a kind of habit, the usefulness of this explanation will be obvious.

The actual processes taking place in the brain when a person thinks or learns or remembers cannot be observed.  The psychologist is, therefore, compelled to construct an explanation which appears to harmonize with what is known and will give a picture of the possible mechanism.  This he does by representing the process as travelling along a path in the brain.  The existence of the path makes it easy for the next mental process to travel the same way.  Like a track across a common or a path in the snow, the brain-path is a line of least resistance.  The facility of habits is due to the fact that the path is made easier each time it is used. What is called "motor memory" or habit is an example of this process of path-making: the child cart repeat the multiplication table until it is an automatic process requiring little or no attention.  On the other hand, when a particular multiple (9 times 7 is 63) is wanted, the path is easily found, and the knowledge revived.



The nerves which have been described above are integrated or brought into union at different levels according to the degree to which the organism is developed. The less developed organisms are those in which the reflex arcs are relatively independent, and stimulation of one segment produces motor response limited to that segment.  As the development progresses, these arcs become more complex, because branches front the axons run out and meet at other points.  The effect of this more complex arrangement is that the movement of an [p. 45] [figure] [p. 46] impulse through the original are ·can be checked (inhibition) and either wholly restrained or redirected. Thus there comes into existence, over and above the primary arcs, a more complex system of arcs forming the lowest or "spinal" system of arcs, so called because the whole of the are is contained within the spinal cord.

The next higher level is formed by further developments from the spinal ganglia.  These new outgrowths are grouped together at the anterior extremity of the spinal cord and form the rudimentary brain in animal development or the basal ganglia of the brain in all vertebrates.

Finally, the highest centres are developed, and the brain is produced to form the centre of complete integration.

The arrangement of the nervous system in different levels is important for various reasons.  It provides a clear meaning for the phrase "mental development" when this is applied in comparative psychology to the relation between animals and man.  A comparison between different animals can be made by observing the extent to which the total mass of matter is developed at the foremost end of the spinal cord where the arcs of the second level are united.  A comparison between human and other animal organisms shows a similar difference in the development of the brain both in its total mass and its special parts.  It is also true that arrested development or retardation shows the same features, for the ·first or lowest level may be completely constructed, but the failure to progress is shown by relatively poor co-ordination if the mechanism of the second level is not adequate, or by intellectual inferiority if the mechanism of the third level is not normal.

The experimental method shows the same results. [p. 47] If the higher centres are destroyed, the lower centres will continue to function and will revert to the simpler forms of combination of reflex arcs.  Various stages of intoxication afford a good example of the facts, as the obvious signs are the different degrees to which control over the ideas, perception, and movements is progressively lost.



Another aspect of this doctrine of levels is seen in its relation to learning.  The primary level is so firmly established in the structure of the organism that its [p. 48] functions do not require to be learned at all.  The functions of this level are performed from the beginning of life as the hereditary endowment of the organism.  The beating of the heart, the action of the stomach, and reflexes in general belong to this primitive class of organic functions.

The functions of the second level are less well established.  The general or diffused character of the movements in young animals and in children is due to the spread of stimulation through different channels with no fixed or orderly connection to bring about a definite selection of preferred paths.  The effect of training is to select and establish the most suitable lines of discharge, giving a preference to some and inhibiting, others. This may be observed in the way a child learns to write. The first efforts are diffused over the neural centres, so that many muscles (hand, arm, face, tongue) are brought into action. As the operation becomes more defined and the correct habits are acquired, the surplus movements are inhibited. This "economy of effort" is made more and more complete, until only the absolutely necessary movements are employed.

Finally, the highest degree of organization and orderly conduct is reached when the functions of the third level are established.  When, for example, a particular order is given, its meaning is properly understood, the sensory and motor centres are properly co-ordinated, and the whole behaviour is the result of a complete unification of the various factors, we may call the result a complete form of organic response. In physical training, after the individual movements have been acquired, a single word of command is enough to begin the whole exercise. If we consider the complexity of the response when the pupil is told to draw an object from memory, it will not be difficult to understand the importance of the selection [p. 49] and co-ordination which is characteristic of this highest level.



The autonomic system is an important part of the bodily mechanism.  This consists of ganglia (groups of nerve-endings) which lie outside the spinal cord and form chains of neural centres running parallel to the spinal cord.  The word "autonomic" is used to cover three large groups of nerves--the cranial or upper part, the sympathetic or intermediate part, and the sacral or lower part. This division is not important for the present purposes, but is mentioned because the term "sympathetic system" is often loosely used for the whole autonomic system.  The word autonomic (cf. autonomy) means "self-regulating". It is sometimes called "involuntary", because it is not controlled by the intellect or rational will.

The autonomic system governs the action of the smooth muscles, and is, therefore, chiefly concerned with visceral organs and glands. For psychology its importance lies in the fact that it controls the kind of reaction which accompanies  emotional  states  and  constitutes their peculiar quality.  If an animal is excited by fear or anger, several bodily changes take place.  The secretion of gastric juice may be reduced, the heart may beat more violently, breathing becomes deeper, and the adrenal glands are excited to pour into the blood an enlarged quantity of adrenalin. Apparently all these changes are spontaneous forms of adaptation to the situation, and they all serve to heighten activity.  The most quoted case is that of the adrenal glands, because the action of these glands increases the strength and endurance of the creature.  While all these activities [p. 50] are in progress, it is unlikely that the intellectual powers will remain undiminished.

It is, therefore, necessary to remember that, if emotions are aroused, intelligence, control, and forethought will be reduced.  The use of methods which frighten or irritate a child will never be likely to promote understanding or assist the progress of intellectual work.



A few facts about the muscles are important for the student of human action.  Muscles are bundles of fibres covered by a sheath.  They have a power of contracting or relaxing, and this is a persistent quality called the "tone" of the muscle.  The motor-nerve which runs to a muscle gives it the necessary stimulus and causes it to contract sharply; after this it relaxes again, unless some peculiar condition causes a failure in this process, and muscular rigidity ensues.  The muscular mechanism is arranged on the principle of antagonism or balance. The extensors and flexors counteract each other, producing in normal cases a proper rhythm of action: defective action of this mechanism produces jerky and erratic movements.  The muscles of the skeleton are classed as "striated" or striped muscles, because the fibres in the sheath are marked with alternate light and dark bands across them.  The visceral muscles, such as those of the alimentary canal, arteries, bladder, and other organs, have no such stripes, and are called "unstriped"  The heart is a muscle constructed of muscular tissue in which the fibres are lightly striped, but are not made exactly like those of the ordinary striped muscles.

For general purposes it is sufficient to remember that muscles are classed as striped or unstriped: that these practically correspond to skeletal and visceral muscles: [p. 51] that, accordingly, the striped muscles are employed for voluntary and the unstriped for involuntary action. The skeletal muscles  act  as levers:  the  visceral muscles produce action in the walls of the organs in which they are found, causing contraction or expansion by which the contained space is decreased or increased.  Pallor, flushing, quickening of the pulse, and the contraction of the large intestine are examples of the activity of visceral muscle.



Emphasis has been laid throughout this account of the body on the organic character of action, which means chiefly that all the parts co-operate and depend on each other for support.  This feature is shown even more clearly when we consider the glands.

The glands commonly known are masses of cells which produce characteristic secretions and periodically discharge the secretion through some duct.  To this class belong the liver (bile), the pancreas (pancreatic juice), lachrymal or tear glands, sweat glands, etc. about these glands nothing need be said beyond noting that they are "organs of response", and that they take a significant place in a, person's total reaction.  They may be excited by external stimuli, for example, tear gas; or ;by internal stimuli, for example, bad news.  The relation between these kinds of stimuli and the response is an interesting problem to be discussed in relation to emotions.

The glands mentioned above have ducts: others are not so constructed and are called ductless.  Modern physiology has made so much progress in the study of the ductless (endocrine) glands, that they form the subject of the special science called endocrinology. These glands operate on the whole system, because their [p. 52] cells produce chemical compounds in minute quantities which are taken up into the blood-stream as it flows around them. The products of these glands are called "hormones" or excitants, and they operate on the body by accelerating (or sometimes depressing) the activity of other organs.

The chief ductless glands are the thyroids, parathyroids, pituitary, pineal, and adrenal glands.  The thyroid secretion affects the whole growth of the body: defective thyroid activity produces the type called Cretin -- stunted, thick-skinned, and permanently infantile. The adrenal glands which produce adrenalin are specially interesting, because this secretion has the power of increasing muscular activity and endurance. Biologically it serves to make animals more capable of endurance in the excitement of rage or fear and in the acts of fighting or escaping.  The other glands have important functions for which we may refer inquirers to the works on general physiology.    The details are not essential to the present purpose.  If the reader will form, from what is here said, a mental picture of the complete machinery of the body, analyzed into nerves, muscles, and glands, that picture will serve to make intelligible the physiological  references which inevitably occur in the explanation of psychological functions.