Physiological psychology

Preface

It is now generally recognized that all students of the mind should have some knowledge of the structure and functions of the nervous system. Unfortunately it is not usual, and in many cases it is not possible, for students of psychology to make that thorough study of the nervous system which is desirable, and even those of them who are fortunate in this respect find some difficulty in bringing their physiological and anatomical knowledge into relation with that which they acquire by the study of works on psychology. The writer of this little book has therefore sketched in broad outlines the structure and principles of action of the nervous system and, assuming on the part of his readers some acquaintance with the general principles of psychology, has endeavoured to show how each of these two bodies of doctrine, the physiological and the psychological, supplements the other, deepening our insight into the processes that result in the actions of men and animals, and how the conjunction of the two kinds of research brings before us a number of problems of the deepest interest that remain hidden so long as we confine our attention to one or other of these sciences. These are the problems of the infant science of psycho-physics, the science that investigates the relation of body and soul. The book should therefore not be read alone, but should be studied concurrently with such standard works as Prof. James7 Principles of Psychology and Prof. Stout’s Manual of Psychology , and the reader who has made no special study of the nervous system should at least have at hand some such description and figures of the gross anatomy of the nervous system as are to be found in any one

VI

PREFACE of the many excellent text-books of human physiology or anatomy.

The scheme of the nervous system and its functions that the writer has sketched is based upon a careful study of the best authorities ; but it is only fair to warn the reader that great differences of opinion upon many fundamental principles still obtain among these authorities. It seemed better to adopt, and apply consistently, a workable scheme of the elementary nervous functions than to confuse the reader by frequent references to these differences of opinion.

Contents

INTRODUCTION ......

CHAP.

I. A ROUGH SKETCH OF THE STRUCTURE AND FUNCTIONS

CHAPTER I A Rough Sketch of the Structure and Functions of the Nervous System

The conduct of a man or animal, in the wide sense in which the word conduct is used in these pages, is the sum of the movements of the parts of his body, and all such movements are effected by the contractions of muscles. By means of such contractions he transports his body from place to place, manipulates the things of the external world, and communicates with his fellow-creatures by gesture, speech, or other signs. Some of these movements, for example, those of speech and gesture, have no immediate relation to the things of the physical world, but others are adjusted, often with marvellous nicety, to produce effects on those things. Movements of the latter kind are guided by impressions made on the sense-organs by the things to which the movements are adjusted. This guidance of the movements by impressions made on the sense-organs is rendered possible by the nerves which connect the sense-organs with the muscles.

The simplest kind of nervous system, possessed by such creatures as the sea-anemone and the jelly-fish, consists of a few nerve-fibres connecting a few very simple sense-organs with a few simply-arranged muscle-fibres, the nerves being more or less intimately connected to form a network or plexus. When any physical change of a kind and intensity capable of stimulating the sense-organ, when, for example, a solid object suddenly comes in contact with one of the sense-organs, a physical or physico-chemical change, which we call the

H nervous impulse, is transmitted from it along the nerve to one or more of the muscles, and excites in them chemical changes which cause them to contract and so withdraw that part of the body from contact with the object. That is a simple reflex action carried out by means of a simple sensori-motor nervous arc in response to a stimulus applied to a sense-organ. It is the fundamental type of all nervous action, and it would seem that the nervous system of any one of the higher animals, even of man himself, consists essentially of such sensori-motor arcs conducting nervous impulses from the sense-organs to the muscles.

The complex nervous systems of the higher animals may be regarded as having been evolved from this very simple type through ( 1 / a great increase in the number and variety of the sense-organs, many of which become highly specialized for the reception of particular kinds of physical stimulus ; (2) a corresponding increase in the number Fig. i.

of the muscles and in the com- Diagram to illustrate a very simple

plexity of their arrangement;^-^

( 3 ) a more than corresponding face of the body, each directly con- • ' • « l j nected with a muscle-fibre (m m) by

increase m the number and com- a nerve-fibre (* n). plexity of arrangement of the

sensori-motor arcs that connect the muscles with the sense-organs ; instead of being simple paths by which the nervous impulses are conducted directly from each sense-organ to 8ome one muscle or group of muscles, the very numerous sensori-motor arcs become connected together in very complex fashions, so that the impulses originating in any one sense-organ may spread through the whole nervous system, or through any part of it, and be transmitted to all or any of the muscles.

i6

In the higher animals and in man we must distinguish two systems of muscles and two corresponding systems of sense-organs. The one system of muscles is attached for the most part to the bones of the skeleton, and the contractions of the muscles of this skeletal system produce all those movement of the limbs, trunk, head, and organs of speech by means of which relations with the outer world are maintained. In man these movements are under the control of the will, and the muscles of the skeletal system by whose contractions they are effected are therefore called also voluntary muscles. The muscles of the other system are embedded in the organs of reproduction and in the various organs whose function it is to keep the whole body in a state of vital efficiency, to prepare the food for absorption into the blood, to aerate and purify the blood and to drive it to all parts of the body in just such quantities as are needed by each part. The principal of these organs are the viscera of the thorax and abdomen, and the muscles are therefore known as the muscles of the ’visceral system , or, because their contractions are not under the control of the will, as the involuntary muscles. With these muscles must be grouped the other great system of active organs, the glands which secrete the digestive and other juices and regulate the composition of the blood, for they are controlled by the same system of visceral nerves.

The muscles of the skeletal system are intimately connected by sensori-motor arcs with the sense-organs of the surface of the body which are stimulated by the things of the outer world. The muscles of the visceral system are connected by sensori-motor arcs principally with sense-organs that are embedded in the viscera, and are stimulated by movements, pressures and chemical changes in the viscera ; and these arcs constitute a system of nerves that was for long considered to be quite separate from, and independent of, the other larger system, and was known as the sympathetic system. We know now, however, that the two systems of sensori-motor arcs, the skeletal or voluntary and the visceral or involuntary, are intimately connected.

The skeletal muscles are controlled not only by impulses from sense-organs on the surface of the body, but also by impulses initiated in sense-organs that are embedded in the muscles themselves and in the surrounding tissues, the sheaths of the muscles and their tendons and the joint-surfaces of the bones ; these sense-organs, which are known as the organs of the “ muscular sense,” are stimulated by the contractions of the muscles and the movements of the parts resulting from those contractions.

The nerves from these sense - organs join the sensori-motor arcs that connect the skeletal muscles with the sense-organs of the surface, and so

Fio. 2.

Diagram illustrating the functional units, the of each

, segment of the nervous system of a vertebrate animal. C the central nervous system, m a skeletal muscle

CO-Operate with them skeletal and visceral sensori-motor arcs

in determining

sequence and the attached to bone, s the skin covering it, z/a muscular f 1 r l viscus. On the right is shown a skeletal arc with

force Or the contrac- double afferent path from sense-organs of the skin firms df thpcp hukcIpq and from sense-organs of the “ muscular sense ” in

tions ot these muscles, tendon> joint.surface> and muscle . on the left an arc

which are thus under of the visceral system with afferent path from sense- 1 11 organs in and about the viscus.

a double sensory con- s

trol as well as the control of the will.

These are the functional units which with their complicated interconnexions constitute the nervous system. They are represented schematically in the diagram Fig. 2.

A sketch of the way in which the nervous system of man seems to have been evolved from a simple form affords the best introduction to an understanding of the plan according to which the functional elements described above are connected together to form the spinal cord and of the plan according to which complex systems of arcs, connecting sensory and motor neurones, are built upon these fundamental arcs to form the brain. Man is the highest member of the family of vertebrates, all of which seem to have been evolved from a simple segmented creature not unlike some of the annnelid worms or the long-bodied arthropods. The existing vertebrate most nearly resembling in general structure the hypothetical ancestor is the little fish-like creature Amphioxus : other vertebrates of intermediate grades of development, the fish, the frog, the rabbit, the dog and the ape may be taken to represent roughly the series of forms of increasing complexity through which the ancestral line of man has ascended. We may conceive the ancestral vertebrate to have consisted like a worm of a series of segments, each of which was almost a complete organism. Each such segment may be supposed to have consisted of certain groups of muscles, covered by a segment of the skin and connected with the sense-organs scattered over that part of the skin by sensori-motor arcs whose middle parts are gathered together in a little cluster or ganglion ; and within the ring of muscles of each segment we must imagine a section of the alimentary canal, a large pulsating artery, a kidney and other viscera.

The segments are not to be imagined entirely independent of one another, because the alimentary canal is continuous through all of them, their blood-vessels communicate, and the ganglia of neighbouring segments are connected by a longitudinal strand of nerve-fibres. Nevertheless the stimuli affecting the sense-organs of each segment are reflected by simple sensori-motor arcs chiefly to the muscles of the same segment. The animal progresses in the direction of its oral extremity, and the special sense-organs, those of taste and smell, of sight and hearing, become developed about this extremity, since their function is to make the creature aware of the nature of the objects towards which it is moving at any moment. With the development of these special sense-organs in the leading segments the number of sensori-motor arcs in those segments becomes greatly increased, and since the movements of the animal as a whole, i. e. the contractions of the muscles of all segments, must be made to work together for retreat from, or advance towards, the things that affect these sense-organs, the nervous connexions of these leading segments with the ganglia of the remaining segments become multiplied. Hence the ganglia of the leading segments become much larger and more complex than the rest ; special protection is required by them, and special muscles are needed to move these special sense-organs. Thus the oral extremity containing the rudiments of the brain, the special sense-organs and the mouth, becomes the head.

The growing bulk of the brain excludes from the head all visceral organs save the extremity of the alimentary canal leading from the mouth. The regular segmental arrangement of the viscera and of the muscles of the rest of the body becomes disordered by the development of the fore and hind limbs, which require for their varied movements larger and more numerous muscles and more complicated nervous connexions than those of the limbless segments. The stage thus reached is represented by the fishes and the simple amphibians. In the nervous system of man and of all the higher vertebrates we can distinguish the parts which represent the nervous system of the ancestral vertebrate at this stage. They are the spinal cord and basal ganglia of the brain, the latter representing the ganglia of the special sense-organs of the head, the former the chain of ganglia of the body and the longitudinal strand which connects them. These contain, of course, many nerve-fibres which serve to connect them with the later-developed or higher parts of the nervous system, namely, the cerebral hemispheres or great brain ; but apart from these, they consist of sensori-motor arcs, small groups of which are connected together to form simple systems of arcs. The conception of a functional system of arcs is one of great importance. A system consists of a number of arcs so intimately connected that, when the sensory limb of any one of them is stimulated, the impulse spreads to all the members of the system and excites a group of muscles whose contractions produce an orderly movement of some part or parts of the body or, as it is technically called, a co-ordinated movement. The arcs thus combined in simple systems and constituting the spinal cord and basal ganglia are the foundation on which the rest of the nervous system is built up. They may be called the arcs of the lowest or spinal level.

In the next stage of development branches grow out from the sensory limbs of the arcs of the spinal level of each segment, form various connexions with similar branches and return to the motor limbs of the spinal arcs of the same or other segments. They thus form loops upon the arcs of the spinal level and combine them in systems of greater complexity ; by means of these more complex systems more complex conjunctions of sensory impressions co-operate to produce more elaborately co-ordinated movements.

Just as in the primitive vertebrate the middle parts of the arcs of each segment cluster together to form a ganglion in which they are connected with one another by branches, so these loops growing out from the arcs of each segment at a later period of the development are brought together in a single large ganglion which is the rudiment of the cerebrum or great brain, and which appears as an outgrowth from the ganglia of the special sense-organs of the head. These ganglia thus became the basal ganglia of the brain. These loops upon the arcs of the spinal level may be distinguished as arcs of the second or intermediate level. In the vertebrates of an intermediate grade of development such as the rabbit, they constitute almost the whole of the cerebrum.

The third great stage of the evolution of the nervous system is characterized by a great increase of size of the cerebrum, due partly to an increase in the number and complexity of the interconnexions of the arcs of the intermediate level, but chiefly to the development of a great mass of nervous elements which become organized into arcs combining the neural systems formed by the elements of the first and second levels into systems of still greater complexity. These arcs of the third or higher level are related to those of the second level in much the same way as these are related to those of the first or spinal level. In the higher animals, in the dog, and still more in the ape, the arcs of the second level derived from each of the great systems of sense-organs are grouped together in the cerebrum forming the parts known as the sensory areas (see Fig. 3) while the laterdeveloping arcs of the third level are intricately interlaced forming areas, known as the association-areas, surrounding these sensory areas and separating them from one another. In such animals the arcs and systems of arcs of the spinal level are congenitally organized in each individual, u e . the details of their structure and interconnexions are completely determined by heredity and are incapable of modification, save perhaps in a very small degree, in the course of the lifehistory of the individual. The arcs of the second level and their interconnexions must also be regarded as almost completely organized congenitally. But it is characteristic of those of the higher or third level that their organization, their interconnexions by means of which the simpler neural systems of the first and second levels are combined to form systems of great complexity, is congenitally determined in a very partial degree only, and is principally determined in each individual by the course of its experience. The arcs of the higher level thus constitute the physiological basis or condition of docility, the power of learning by experience.

In the brain of the child the arcs of the first and second levels very early become organized in systems that take form independently of the child’s experience ; but in addition to these there is present at birth a vast number of nervous elements which become only gradually organized, modifying and combining the congenital systems in extremely complex systems which constitute acquired dispositions to modes of action peculiar to the individual. The number of the elements of this nature is so great that the areas of the cortex of the cerebrum in which they predominate, those which surround

Right cerebral hemisphere of man seen from the outer (the upper figure) and from the median side. The sensory areas are dotted. The part marked motor area is the kinxsthetic sensory area. 'Flechsig’s figure as modified by Schafer.)

the various sensory areas and are known as the association-areas, are considerably more extensive than the sensory areas, which in the higher animals make up the greater part of the cortex. The greater relative size of the brain of man is chiefly due to the great development of these parts. The very great capacity for learning by experience, rendered possible by this vast mass of nervous elements not congenitally organized, distinguishes the mind of man and raises it immeasurably above that of the highest animals.

This description of the nervous system as consisting of sensori-motor arcs of three principal levels is of course a very much simplified schematic view, and it is probable that future research may enable us to distinguish a hierarchy of levels among the arcs that we have grouped together as constituting the third level. Nevertheless it will form a useful basis for the exposition of nervous functions, and its substantial truth is borne out by three kinds of observation : (1 ) The comparison of the brains of vertebrates of different levels of development ; (2) the order of development of the parts of the human brain, the three levels attaining structural perfection in the order from below upwards ; ( 3 ) the loss of function in the reverse order under the influence of disease, noxious drugs and general constitutional decay ; for the oldest or most primitive parts, having attained the greatest fixity or stability of organization, offer the greatest resistance to destructive influences, while the latest acquired structures are less stably organized and therefore give way most readily under stress of any kind.

Before describing in more detail the functions of these nervous arcs and the parts they play in our mental life, it is necessary to examine more closely the nature of nervous tissue and of the processes that occur in it.

Chapter II

The Minute Structure of the Nervous System and the Nature of the Neural Processes

The Neurones . — The nervous system is made up of an immense number of nerve-cells bound together and supported by sheaths and strands of connective tissue and finely branching cells, the neuroglia-cells. Each nerve-cell, or neurone as it is now commonly called, is, so far as the maintenance of the vital processes of nutrition and growth are concerned, a self-contained individual, not an independent individual but a member of a very complex society, the cells of the whole body ; and its welfare, like that of any member of a highlycivilized community of persons, depends upon the continuance of harmonious co-operation among the members of the society.

The neurones are of various shapes and sizes, but each one consists of a nucleus, which with a certain amount of protoplasm surrounding it constitutes the cell-body,1 and one or more delicate protoplasmic processes continuous with the cell-body. In many neurones one of these processes is much longer than the rest and is called the axis-cylinder process or axon. All the nerve-fibres of the peripheral nerves are axons of neurones. The axon, in most cases, has for its protection a delicate sheath of fatty matter known as the medulla , and in

1 The cell-bodies were formerly spoken of as the nerve-cells or the ganglion-cells, and the false implications which the word nerve-cell has thus acquired justify the use of the word neurone to denote the whole cell.

many cases it is of great length, those, for example, which run from the spinal cord to the ends of the toes of a tall man are several feet in length. Some axons give off at short intervals a number of branches or collaterals , which connect them with numerous other neurones. The other processes of the body of the neurone are generally shorter and much branched ; they are known as dendrons because in many cases a single large process of this kind divides, like the stem of a tree, into many branches ending in a great number of twigs.

Those neurones which have long axons have very large cell-bodies, for the cell-body presides over the nutrition of the whole cell. The large cell-bodies of the motor neurones lie in the anterior horns of the central core of grey matter of the spinal cord and in the corresponding parts of the basal ganglia of the brain. Their axons pass out directly in bundles which form the anterior roots of the spinal nerves and corresponding nerves of the head. The cell-bodies of the sensory neurones cluster together in the ganglia of the posterior roots of the nerves, which roots consist of bundles of the axons of these cells. It is thus the peripheral ends of the axons of the &nsory neurones which receive the physical impressions of the outer world. All the other neurones, with the exception of a relatively small number belonging to the visceral system of nerves, are wholly confined to the central nervous system, and make up the paths and systems of paths of extreme complexity that connect the sensory with the motor neurones. Some have large bodies and long axons connecting widely separated parts of the central nervous system, as, for example, the pyramidal cells of the cerebral cortex which send down their axons to the various segments of the spinal cord. Others are small and have short axons which are hardly distinguishable from the dendrons.

It has been assumed by some authors that the dendrons always serve to receive the stimulus from other neurones and to conduct impulses towards the cell-body, and that the axon always conducts impulses from the cell-body, but exceptions to this premature generalization are known, and it is mentioned here only that the reader may be warned against accepting it. The neurones which make up the retina and

W M M

Illustrating various types of neurones or nerve-cells. (After Sir M. Foster.) a a axons, dd dendrons, bb terminal twigs that form synapses with the similar branches or the bodies of other cells.

the optic nerves, and those which constitute the olfactory sense-organ are exceptions to the rule that the sensory neurones receive the physical impressions of the outer world upon the peripheral ends of their long axons, for the cell-bodies of those neurones lie in the sense-organs, and are connected with the brain by chains of neurones. These sense-organs are in fact outgrowths from the brain and are, morphologically considered, parts of the brain.

Although the neurone is a vital or trophic unit, it is not the functional unit of the nervous system, for, as we have already seen, the functional unit is a sensori-motor arc through which the nervous impulse is conducted from sense-organ to muscle, and the simplest sensori-motor arc consists, in the higher animals and man, of at least two neurones joined end to end, a sensory or afferent and a motor or efferent neurone. Even a reflex arc of the spinal cord may consist of a chain of several neurones thus joined end to end, and in the complex arcs of the higher levels the number thus joined in series may be still greater. Besides being joined end to end the neurones of the various arcs are also connected in complex ways by collaterals and by the branches of the drendrons. An understanding of the nature of these junctions of neurones is, as we shall see, of the greatest importance for the physiological psychologist.

The Synapses. — The junctions between neurones are conveniently named synapses. The microscopic investigation of the synapses is unfortunately so difficult that, in spite of much labour devoted to it, our knowledge of their structure is still very imperfect. A simple kind of synapse is formed by the division of the end of an axon, or of one of its collaterals, into a number of fine twigs, that surround the cell-body of another neurone and terminate in tiny knobs lying close to, or perhaps in contact with, that cell-body. Others are formed by the minute terminal branches of an axon or a collateral becoming intertwined with the branches of a dendron of another neurone or, in some cases, perhaps, with similar fine terminal twigs of the axons or collaterals of another cell. In many cases the terminal twigs of several axons surround one cell-body or are intertwined with the dendrons of a single neurone. It has been generally asserted that there is no continuity of substance between the neurones that are joined at a synapse, and although recent researches seem to show that there is protoplasmic continuity in certain cases, we may confidently believe that in the great majority of synapses there is no simple continuity of substance, but a breach of continuity, or a difference in the nature of the nervous substance, which prevents the passage of the nervous impulse in the form in which it travels along the nerve-fibres, and renders that passage more difficult. This peculiarity of the synapses may be conveniently expressed by saying that each synapse presents a certain resistance to the passage of the impulse. And there is good reason to believe that in many cases the apposed parts of the two neurones are separated by a thin layer of a very highly specialized substance, and that to the presence of this cementing substance is due not only the greater difficulty with which the nervous impulse is transmitted from one neurone to another across the synapse than from part to part of each neurone, but also certain other peculiarities of the transmission of the impulse in the central nervous system.1

The Neural Processes . — Each neurone, like every other living cell, is the seat of metabolic processes of two kinds, the katabolic and the anabolic . The katabolic process consists in the partial decomposition of some of the very complex and unstable molecules that are the essential constituents of its substance. By such decomposition some of the chemically stored or potential energy of the molecules is converted into the free energy by means of which the cell discharges its social functions. The anabolic processes consist in the reconstitution of the complex molecules by means of the oxygen and the food-substances that the cell absorbs from the blood. It is the social functions, the part which the

1 The evidence which points to the existence of such inter-neural substances is presented in a paper in Brain , vol. xxiv., entitled “ The Seat of the Psycho-Physical Processes. n neurone plays in the economy of the whole organism, with which we are concerned in these pages. So far as we know the social function of the neurone is simply to respond to any stimulus applied to it with a katabolic change that gives rise to a “ nervous impulse ” and to conduct this “ impulse’7 from part to part throughout the length of its substance. As to the essential nature of this “ impulse ” we are still ignorant. Some authorities are inclined to regard it as a purely physical change, such as the conduction of heat or electricity in a wire or of a fluid-wave of pressure in a pipe, but it is possible that the conduction of the “ impulse ” through each part of the neurone involves chemical changes of the same kind as those which initiate it at the spot stimulated. The only kind of change of which we have direct evidence is a change of the electrical state of each part of the neurone as the “ impulse ” passes through it. When a nerve is stimulated in the middle of its length a wave of electrical change travels in both directions along it from the spot stimulated at a rate of several yards a second, and for this and other reasons we believe that the impulse is conducted through the substance of the neurone equally well in all directions. In the motor neurones the “ impulse ” commonly takes the form of a rapid succession of such waves of change, but it is probable that in the sensory nerves the “ impulse ” may consist in a change constantly transmitted from part to part so long as the stimulus is applied.