The Nervous System and Its Conservation

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Preface

There are several excellent books devoted to the anatomy of the nervous system, there are others which treat of its physiology in an illuminating way, and others still which deal with its hygiene. If the present volume is not to be an impertinence, it must be justified by its attempt to combine some elements of the three. The intention has been to present no more anatomy than is essential to an appreciation of the correlated physiology and to subordinate the physiology, in its turn, to the teaching of hygiene.

In the chapters which deal with structure and function the writer has tried to gather his material from authoritative sources. In those occupied more largely with hygiene, personal judgments have doubtless found expression. But the desire has been not so much to dictate in a dogmatic spirit as to open important matters to fuller discussion.

Many associates have contributed to the making of this book. From the number it is just to mention Professor Ernest G. Martin, of Harvard, whose suggestions have been numerous and valuable.

P. (jr. b.

November^ 1914

399490

Chapter I Introduction Tissues

Before the modem microscope was available, anatomists and physiologists recognized in the make-up of the human body various types of constituent substance or fabric. The different kinds of material recognizable were few in number and easily assigned to certain large classes. Comparison of one part of the body with another showed that the same forms of organized substance were found over and over again. The several varieties of material distinguished came to be known as tissues.

In the hand, for example, we find the skin, the muscle, and the bone, representatives of kinds of tissue contrasted in function, in physical character, and in chemical composition. Closer study of the hand would add other members to the list. When we consider the tissues with regard to the intensity of the life-processes taking place in them it is clear that this is anything but a constant quantity. In some cases we have to do with structures which are essentially passive in their nature and which might conceivably be replaced by non-living substitutes. This is true of those tissues which are described as connective. Their service is mechanical; they unite and support other tissues. Bone is a conspicuous example of

12 tKlS NERVCfeB 8Y8l1fl&i;ANb ITS CONSERVATION

this class, and very much as the skeleton determines the general form of the body, so each organ is to be thought of as possessing a minor framework of material which behaves for the most part as though it were not really alive.

It is usual to place the tissues in four large groups. Besides the connective, to which reference has just been made, we speak of the epithelial, the muscular or contractile, and the nervous. Epithelial tissvss are those which form surface layers, either internal or external. They are the linings and the coverings of the organs. The skin, or at least its superficial portion, is an obvious example. While all the epithelia answer to the description of "coverings or linings," they vary widely in other respects. Under the general term are included structures as unlike as the calloused sole of the foot, the exquisitely clear cornea of the eye, and the excessively thin partitions interposed between the blood and the air in the lungs. Judging the epithelial tissues by their share in the total of the life-processes, we may say that they have both passive and active parts to perform. In some places their function is merely protective; in others they have well-marked chemical activity. Secretion and absorption are epithelial duties, and neither the one nor the other could be carried out in its fulness by membranes not living.

Contractile tissiies are those which originate the movements of the parts of the body. Organized into large masses, they form the familiar muscles. The heart is mainly composed of a peculiar contractile material which is responsible for its beating. The same property is manifested by elements found in the walls of the alimentary canal, the blood-vessels, the urinary bladder, the uterus, and other hollow viscera. It is evident that the contractile tissues are eminently *^alive" — that no lifeless substances could be expected to behave as they do. Three principal varieties are recognized, conveniently distinguished as the skeletal, the cardiac, and the visceral

INTRODUCTION 13

— ^names which go far to denote the region where each is discovered.

The nervous tissues are far less conspicuous in an anatomic sense than the three orders just enumerated. Let us imagine for a moment that each of the four divisions in which we have placed the tissues could be presented separately to view. Four human figures would appear before us. That one composed wholly of connective tissue would consist of the skeleton as a basis, and would show, in addition, an indication of each organ in outline, with an internal system of interlacing strands. It would include the ligaments which join bone with bone, the cartilages which also occur about the joints as well as in some other localities, and the tendons uniting muscles with bones. The muscles themselves would be blocked out in this picture by the surface membranes which they possess and by webs of non-contractile material within.

The second figure would retain the complete contour of the human form, inasmuch as it would have the skin. If it were transparent enough the cavities and external surfaces of the viscera would be seen defined in their entirety, but with spectral lightness. The alimentary canal would be traceable as a winding tube of great delicacy within another still more diaphanous. The blood system would be mapped in similar fashion. The interior arrangement of each gland would show a tree-like pattern of bewildering intricacy.

The third figure, consisting of the contractile tissues, would be fragmentary and discontinuous. To conceive of this apparition as keeping its shape calls for an extra stretch of the imagination. It would supply the chief bulk of the muscles and a fraction, missing in the other manikins, of the alimentary, circulatory, and reproductive systems. In a scattering way contractile tissue occurs even in the skin, where its presence makes possible the "goose-flesh'' reaction. Although this third figure would be less coherent than the others, it would greatly exceed them in weight.

What would remain for the fourth member of this uncanny series? It has been reserved for a reconstruction of the nervous tissues. A distant view of it would give the impression of a ghostly form with a more substantial axis expanding above into a bulbous mass in the head. On closer approach the axis would prove to be the spinal cord, the enlargement above would be recognized as the brain, and the filmy haze which completed the figure would, resolve itself into many nerves^ branching strands proceeding fyom both brain and cord and terminating in a cloud of finest filaments. These filaments would be found to reach almost every part of the body (Fig. 1). A large proportion would be seen to end near the external surface; a great many could be followed to localities previously determined to correspond with the skeletal muscles; still others would be manifestly adapted to make connections with blood-vessels, glands, and the contractile elements of the viscera.

By way of recapitulation the following statements may now be made: The human body — or the body of any animal of a highly, organized type — owes its relative constancy of form to an inactive supporting system, the connective tissues. It owes its ability to move to the contractile tissues. The epitheUal tissues are those which establish the boundaries between the organism and the environment; through them exchanges of gaseous and dissolved substances take place. The service of the nervous tissues remains to be indicated. The task is to occupy us throughout the present volume. But a condensed preliminary discussion is desirable.

If a word is sought which shall convey a general idea of the work of the nervous system, it is probable that the term coordination will suggest itself. The nerves serve to place various parts of the body in communication. This statement must be followed at once with the note that the communication afforded is never directly from organ to organ^ but always by way of the axial part of the system. Thus, there is no unbroken nervous high-

Introduction

16 THE NERVOUS SYSTEM AND ITS CONSERVATIOxV

way from the stomach to the heart, nor from one hand to the other. Yet the stomach may influence the heart, the central part of the mechanism transmitting the eflfect. The f midamental plan of the structure involved is that of a tree, and just as an ant cannot pass from one of the main branches to another without visiting the trunk, so all physiologic reactions employ a central link.

Coordination implies order, eflSciency, and timeliness in the operation of a machine. These desirable qualities could not be realized in the living machine if it were not for the nervous system. Coordination is akin to cooperation, and the purposeful working together of all the parts for the advantage of the whole is accomplished through the aid of nerves and nerve-centers. A word which is now frequently heard in this connection is integration. The nervous system may be said to confer integrity upon the organism. This is to say that the unity or oneness of the animal is almost wholly dependent upon this feature. It is not entirely so, for we are realizing more and more that chemical products passing from organ to oi^an have much to do with the coordinating of activities. If the nervous tissues could be instantly eliminated from the body of a dog without doing other injury, the life processes would not immediately cease, but they would become local and partial. They could no longer be combined to the advantage of the animal. The nervous system confers upon the associated tissues the right to rank as an individiial.

It is scarcely possible to use the word "individual" without introducing the idea of a self-conscious being. One's own nervous system is, in part, demonstrated to one through personal consciousness. This is the field of psychology. The physiologist is more properly a student of the objective than the subjective, and he is occupied for the most part with observing the reactions mediated by nervous systems other than his own. Yet it may be claimed that there is a considerable borderland common to the two branches of science within which it behooves

INTRODUCTION 17

both the psychologist and the physiologist to make ventures. It is fortunate, nevertheless, that a great many facts can be noted and discussed without any reference to supposed mental states in animals.

We can deal with all the reactions brought about through the nerve-centers in the simpler animals without assuming psychic accompaniments, and the same method carries us very far with our attempted analysis of nerve physiology in the higher forms. A man cannot regard his own brain in quite the same way in which he regards his liver, but when he is bent on studying the living nervous system in a frog, a cat, or even in another human being his immediate concern is with what it does — not with what is felt while the action is in progress. Eventually he is obliged to consider sensation in connection with his other problems, but it is well to defer as long as possible the admission of psychologic elements.

In accordance with what has been said, an objective treatment of the nervous system will be pursued for the present. The nerves will be looked upon as conductors of energy. The chief source of this energy will be recognized in the external forces which are brought to bear upon the organism and which are termed stimuli. The destination of the currents of energy will be found in the contractile and secreting tissues whose activities they modify. Life that is not one's own is manifested through movement, heat production, and secretion; it is very rarely that we are obliged to take other responses into account. Attention must be directed to the fact that nervous tissues separated from others can scarcely give clear evidence of their internal changes. Muscles and glands are required to testify to what is passing in the governing mechanism. (Except for certain electric symptoms of very slight intensity we should not be able to decide whether a detached portion of a nerve-trunk were Uving or lifeless.)

1 ♦

18 THE NERVOUS SYSTEM AND ITS CONSEKVATION

RECEPTORS; ADJUSTORS; EFFECTORS

These three words have recently been used to develop a most simple and helpful outline of the general work of the nervous system. Whether we consider a worm, an insect, a reptile, or a mammal, three departments of organization with functions of three orders can be indicated. The receptors, as the name implies, constitute that department of the system which is subject to external influences. Receptors are wrought upon by pressure, by changes of temperature, by chemical agents in solution. In the more highly developed animals they include particular sense organs like the eye and the ear. Such organs translate vibratory energy of the ether and the air respectively into the form of energy which nerves convey.

It is to be noted that receptors may be strictly part of the nervous system, as where naked endings of the fibers lie exposed to contacts, or they may be mechanisms made of other tissues serving to transmute and apply the energy of the stimulus. Where the receptor does not constitute a part of the nervous system the path from it to the central axis must be given a special name, and it is spoken of as a sensory or afferent path.

The effectors upon which the nervous system plays include the contractile and glandular tissues. The effectors are always distinct from the nervous system, and the fibers stretching out to them form efferent paths. The third class of working units, the adjustors, are those within the nervous system which mediate between the receptors and the effectors, or, more accurately, between afferent and efferent pathways. It is in the realm of the adjustors that the utmost complexity of organization is possible. The variety of reaction so typical of animals which we call **high in the scale" has its physical basis here.

Parker,^ of Harvard, has written most suggestively of

* G. H. Parker, 'The Origin and Significance of the Primitive Nervous System," Proceedings of the American Philosophical Society, 1911, vol 1, No. 199.

INTRODUCTION 19

the probable sequence in which the three orders of elements have acquired their position in relation to the nervous system of standard type. According to his hypothesis there may have been a certain lowly condition — possibly realized in sponges — in which contractility was displayed only in response to distinct and direct stimulation brought to bear from the outside world upon tissues having the power of movement. No separate and definitely nervous tissue would be requisite to apply the stimuli, but if such a transmitter should be evolved it would be evident that an efferent element had come into existence. Parker thus conjectures that the efferent is the most primitive feature of the three.

In the course of development the submergence of the contractile tissues below the exposed surface of the body would make it necessary that a receptor should be added. Its function would be to maintain relations between the exterior and the hidden effector. One receptor joined with one effector would form a sufficient means for bringing about one, and only one, reaction. The arrangement does not admit of any flexibility. It is the simplest example of what is called a reflex arc. With the addition of intermediate links between the receptor and the effector departments the reaction becomes less and less predictable. This is equivalent to saying that it becomes more and more like the actions of the higher animals — subject to reinforcement, suppression, or modification under the influence of changing circumstances. Many illustrations will be given later which will serve to clear up this matter.

Excitation and Inhibition. — The common conception of the function of nerves is that they arouse activity. This is essentially true of those which stretch out from the central axis to the skeletal muscles, and which call forth the evident movements of the limbs, the play of facial expression, etc. The muscles involved in such movements are relaxed and paralyzed when their nervous connections are interrupted. It is because this fact is so prominent that one may easily fall into the error of supposing that no process of contraction or secretion takes place save as the nervous system initiates it. This is very far from being the case.

All the branches of the nerves which enter the substance of the heart may be cut, but the heart will continue to beat. Hence, we cannot believe that each beat of the heart is dictated by the efferent elements of the coordinating system. The power to make rhythmic contractions is resident in the organ itself; we say that it is automatic. In a less striking, but still positive, way other organs display a tendency to activity when removed from central command. This is demonstrable for the stomach, the intestine, and the urinary bladder. The contractile tissues which enter into the make-up of these viscera must be said to have a degree of automaticity.

When an organ has this disposition to be active on its own account it will be seen that the nervous system may exert a twofold influence upon it. The native, local process may be accelerated beyond its average rate of progress, or it may be restrained more or less radically. We say that tissues which possess the automatic property may exhibit either excitation or inhibition when the nervous system modifies their inherent tendency. It is probably just to claim that both with the heart and with the alimentary tract inhibitory effects are more frequently witnessed than are those which are clearly excitatory. If one is inclined to question this, on the ground that quickening of the heart-beat in exercise is a daily experience, it may be answered that this quickening is known to be to a great extent the withdrawal of an inhibition previously operative. Withdrawal of inhibition is a form of counterfeit stimulation which will repeatedly call for consideration in later paragraphs. The slowing of the heart after exercise may be a more positive instance of nervous inter-' vention than the hastening of its action, which seems so much more impressive.

As for the alimentary canal, it has become well recognized that its movements are actually observed to better

INTRODUCTION 21

advantage when it is removed from the reach of central influences than when its nervous connections are intact. It is not unUkely that there are a good many human beings suffering from indigestion of nervous origin who would have less trouble if their stomachs and intestines could be placed quite out of reach of brain and cord and left to work as determined by their own intrinsic properties. According to this view, suspension of the motor activities of the canal is a possibility easily realized. Acceleration may also be effected, but cannot be held to be so common and striking an occurrence.

When the first clear instance of inhibition — ^the case of the heart — was brought to the attention of scientists in the year 1845 it was received with surprise and, at first, with incredulity. All sorts of explanations were offered for the phenomenon, with the intention of proving that it was not a true suppression of activity. In course of time, however, inhibition had to be recognized as a power of the nervous system fully as direct and characteristic as excitation. It is now fair to say, as a concise statement of all which has gone before, that the duty of the nervous system is to translate the energy of external stimulation into nerve-impulses y to carry these to the central axis, and to apply them through the efferent paths either to arouse or to restrain the action of the contractile and secreting tissues. The nature of the energy which is transmitted along the nerves can best be discussed after we shall have obtained some insight into the microscopic organization of the nervous mechanism.

Chapter II The Minute Structure Of The Nervous

TISSUES

Our Introduction began with references to the existence of a few types of tissue in the human body. These types have been famiUar to anatomists for more than a century. A large share of the credit for their recognition and definition belongs to Bichat, a French worker, whose short life of only thirty years (1771-1801) was extraordinary for the extent of its achievements. Forty years after the death of Bichat an important step had been taken toward a more comprehensive knowledge of the way in which living matter is constructed. The doctrine declared at that time is known as the Cell Theory. Its establishment was directly due to a noteworthy improvement in the lenses made for microscopes.

The Cell Theory is to the effect that all forms of life can be demonstrated to exist as single or associated units of a certain standard kind. These units of structure continue to bear the name of cells, though the word became fixed upon them at first as the result of a misconception. It is not logical to use a word which properly means a cavity to stand for a more or less solid mass of material, but this is the actual practice. The biologic cell may be regarded as having the same relation to a cell in the precise sense of the term that a cast has to the mold. In short, the cell is a small parcel of living matter, the smallest which can continue long to have the behavior which we associate with the living state.

An average cell is microscopic in size. Many of those in the body would be found to measure less than jj^ inch in diameter; the red corpuscles of the blood, for example, are only g^^ ^ inch across their circular faces. If there can be said to be a standard of form for the cell, it is the sphere. But this primitive form is merely the point of departure for the widest variations. The units in contractile tissues are always greatly elongated, those of epithelia are flattened, cubic, or prismatic, those of connective tissue are irregular and often quite freely branched. The i

Fig. 2. — DrawJDga like tbe above are almost always made from tissues which have been prepared aad colored by special means to make clear, minute features, o repreaeots an ovum or eeg-cell, the typical cell may be assumed to t«nd toward this spbi^ric form; & is a cell from a compact tissue, to show how mutual pressure produces a faceted or polyhedral form; c is a contractile element such as occuT'' in the walls of the alimeatary canal, it illustrates an eloapated cell; (i is an epitheUal or lining cell of the order found on the mner surface of blood-vessels; this is an example of extreme fattening; e, from the nervous system, exhibits the possibihty of a br&nch' ing development.

cells in nervous tissues are most peculiar of all, and must presently be described at some length (Fig. 2).

When such diversity of form is considered the student is disposed to ask: What have these differing packets of organized substance in common that the same name should be applied to them all? It is hard to say that any single feature is always to be found in them. One which is prominent in the great majority is the nucleus. This is a body within the cell, seemingly more dense than the remainder and unlike the rest in its chemical character. The last fact is proved by the individual reactions of the nucleus toward selected stains or dyes. When tissues are treated with such solutions, the nucleus, which may have been quite invisible in the uncolored specimen, appropriates the stain and stands out in striking contrast to the pale substance which envelops it. Exceptionally there may be more than one nucleus to a cell or, again, none may be discernible. Cells are said to be composed of protoplasnij and the word is often used as synonymous with living matter. Huxley called it the physical basis of life. The particular word cyUyplasm will be found useful to designate the portion of the cell which is external to the nucleus.

The statement that tissues are composed of cells is open to some objection. Tissues are made by cells, but in the course of their development they commonly come to contain material of a sort which cannot be described as cellular. Such material is conveniently termed "intercellular substance" and it is probably always lifeless. Sometimes it is scarcely recognizable, while at the other extreme it may be so abundant as to constitute the main bulk of the tissue and to determine its physical properties and its value to the animal. Thus, bone is serviceable because of the intercellular lime salts which give it rigidity. The cells have come to occupy a very inconspicuous position, and though they continue to exert a needed influence upon nutrition, they are not at all concerned from moment to moment in the mechanical function fulfilled by the bone as a part of the skeleton. All connective tissue is distinguished by the high proportion of intercellular substance contained in it. The clear, glassy concrete of cartilage, the tough, white fibers in tendons and ligaments, the network between the skin and the muscles — these are examples of material which is not made of cells, though it was made hy them.

It has proved far more difficult to gain a satisfactory knowledge of the intimate structure of the nervous system than to master the minute plan of other tissues. Students at the present time are by no means agreed in their interpretation. Fortunately, even an imperfect picture of the microscopic arrangement throws much light upon the physiologic processes observed. From one point of view it may be claimed that the chief office of the nervous M system is conduction. We shall do well to attend first to the elements which are known to carry on this particular duty. These elements are the nerve-fibers; collected in bundles they make the nerves.

A nerve is usually assumed to be a strand directly or indirectly connected with the central system and large enough to be followed by the naked eye as the body is dissected. The fiber is too small to be traced in this way. Fibers bear a relation to nerves like that which wires bear to cables in which they are bound up. It follows that the number of the nerves is of a moderate order, while that of the fibers is expressed in millions.^ Nerves are often seen to subdivide, but when such branching occurs the fibers are merely assorted; they do not fork where the nerve does, but are found in equal numbers in the trunk and in the sum of its branches. This does not mean that nerve-fibers never do subdivide, for there are important instances of such behavior, but subdivision is usually met with either in the central axis or near terminal structures rather than in the nerve-trunks (Fig. 3).

A nerve-fiber consists of a core surrounded by one or two sheaths. The double covering is the more common arrangement, at least outside the brain and cord. The suggestion of an insulated wire is strong. Here, as in the wire, it is certainly the core which is the essential conductor; the sheaths can scarcely be regarded as insulating material in the ordinary meaning of the word, but they are rather calculated to support and nourish the axis-

1 Between 3,000,000 and 4,000,000 has been estimated. Donaldson, "The Growth of the Brain,'' Scott, London, 1895, p. 196.

cylinder or axoUy as the enclosed transmitter is called. Investigation by refined methods has made it probable that the axon can be resolved into a number of very small fibrils. The idea that there may be conducting units finer and more numerous than the fibers as usually recognized opens interesting possibilities.

The protective sheaths are broken into segments at intervals of a millimeter or less and thus present a jointed arrangement. These joints are known as the nodes of Ranvier, and the fact is to be emphasized that they do not interrupt the continuity of the axis-cyUnder. This goes on unbroken like the thread in a necklace. Nerve-fibers attain a great length; the longest are not much shorter

Fig. 3. — Two nerve-fibers shown diagrammatically especially to emphasize the continuity of the axon. A detail, further enlarged, to suggest tibrils in the axon.

than the entire body. They may in other cases be so short as hardly to pass outside a single microscopic field. We must now consider the manner of their beginnings and endings.

In studying the nervous tissues without the aid of the microscope anatomists long ago began to refer to two types which they called respectively gray andwbitematter. Most of the nerve-trunks can be decisivelyclassi;^ fied as white matter, and when we find substance of a similar appearance in the cord or the brain we may conclude that its organization is. much the same as that of the ordinary nerves. We have already seen that nerves are bundles of fibers laid closely together and having a parallel direction. The microscope shows that this is equally the

MINUTE STRUCTURE OF THE NKHVOUS TISSUES 2i