Physiology: The Science of the Body

by Ernest G. Martin

It is hard to overestimate the importance of language to the human race. This is because of the enormous extension it gives to the ability to profit by experience. The lower animals are able to profit by experience, but the only experience that they can get is that which comes to them individually or to another animal which they may observe. Human beings, on the other hand, may profit by the experience of their contemporaries through learning of them by word of mouth, or may profit by the experience of present or past generations as recorded in writing. It is not too much to say that the progress of civilization depends on our ability to profit by the experience of past generations as made known to us in writing. We are further advanced than our remote ancestors, not because we are actually superior to them in mental power, but because we have a much larger background of experience than they had, and this background depends altogether on our ability to use language.

The complex nervous activities that have been described up to this point are purely a matter of the association of memories, or what we speak of as intellectual processes. Besides these our mental life includes what we call the feelings or emotions, and these make up so large a part of our mental life, that we cannot leave the consideration of the brain without saying a word about them. For convenience we may subdivide the feelings into the two classes of agreeable and disagreeable. If we try to get at the meaning of these, we can perceive that the agreeable feelings are fundamentally associated with one of two things: either a condition in which bodily well-being is assured, or conditions associated with the perpetuation of the race. An example of this latter is the pleasure that parents find in the care of children. The disagreeable feelings are fundamentally concerned with the immediate preservation of the body from injury. Thus fear and anger are two of our most disagreeable emotions, and both of these are aroused ordinarily only when danger threatens or when bodily well-being is otherwise interfered with. One other disagreeable emotion, worry or anxiety, is interesting in that it applies to ill-being that is expected in the future, and while it is well to be on guard against trouble, this emotion is really for the most part futile, because the expected ill-being commonly fails to materialize. Not only do the feelings share with the intellectual processes in making up our mental life as a whole, but they also show themselves in certain bodily changes, which are so important that we shall return to them at greater length in the next chapter.

The feelings as well as the intellectual processes are made up of nervous disturbances passing over the brain cells. We do not know whether those which are concerned with the emotions are in different parts of the brain from those that are concerned with the simple associations or not. There are some differences between the two kinds of nervous activity; particularly is this true of the vividness of the memories concerned. Every intellectual process, presumably, results in the formation of some associations that were not previously present, and the memory of the new associations should be and usually is reasonably sharp. Emotions on the other hand, although they may be remembered, do not register themselves in such a way that a new emotion is aroused every time the old one is remembered. It is probably true that for a while after any strong emotion the recollection of it will tend to bring up the emotional state again, but in course of time there is a dulling, so that although we may remember clearly the occasion of the emotion, the feelings are no longer actively aroused.

CHAPTER XII
SOME SPECIAL NERVOUS ACTIONS; SMOOTH MUSCLE AND GLAND CONTROL

IN the last two chapters we have talked about the arrangement of the nervous system and its working as seen in simple reflex actions, and the special activities of the brain which make up the intellectual processes and the emotions. Before we can leave the subject of nervous activity, there are some special kinds of reflexes to be spoken of, and there are also some additional things to say about the working of the emotions. When we speak of special reflex actions we have in mind some of the most useful of our own reflexes. They are not different from the simple reflexes that are described in Chapter X, except that they involve a larger number of sense organs and many muscles, and are correspondingly more complicated. Furthermore, on account of this complication a special part of the nervous system is devoted to carrying them on. Back in Chapter VIII, under the heading of motion, the different types of bodily movement were listed. Chief of these was the act of locomotion; that is, the means by which we get from one place to another, an act which is, of course, of prime importance in maintaining life, since it is concerned both with the securing of food and the escaping of harm. Locomotion is performed in a number of different ways; in ourselves, walking, running, leaping, swimming, riding a bicycle are all forms of locomotion. Any of these acts, as stated in Chapter VIII, requires the accurate timing and accurate grading of the contractions of a great many different muscles. As we saw in that same chapter there are special sense organs which furnish the information upon which this accurate control of the muscles of locomotion is based. We have now to consider the part played by the nervous system in carrying it on. Whenever one starts to walk or to perform any of the other acts of locomotion, nervous disturbances will be set up in great quantity from the organs of muscle and joint sense in the parts of the body that are active, and also from the organs of equilibrium in the ear. It is necessary that the whole volume of nervous disturbance that is pouring into the central nervous system from these sources be translated into a series of streams of disturbances passing out over the motor nerves to cause the muscles to work just as they should in performing the act. All this is reflex in the strict sense. That is to say, the muscular action follows the arousing of the sense organs with no more delay than is necessary for the passage of the nervous disturbance. The work of transforming the incoming stream into a suitable outgoing stream is performed by a part of the brain about which little has been said thus far, namely the cerebellum. This is connected with other parts of the central nervous system in such a way that pathways from organs of equilibrium and from the organs of muscle and joint sense lead into it. From it in turn come a great many connecting nerve cells whose axons pass along the spinal cord and form nerve junctions with the motor nerve cells; and so nervous disturbances coming from the cerebellum can arouse the muscles to activity. We all know that walking or any other locomotor act, after it has once been learned, is carried on successfully without our performing any intellectual work in connection with it. What we do have to do is to start the act, and this is done through a discharge from the cerebrum; either some sense organ is aroused and sends a disturbance into the cerebrum, which arouses in us an association which indicates the necessity for going somewhere, or else a pure memory not stimulated from outside has the same effect. In either case the nervous disturbance passes over from the cell where the association resides to the motor area, and from there down to start the act of walking; but as soon as it is started, the reflex takes hold and carries it on without any further thought on our part except as some additional associations enter to cause us to change or perhaps to stop the action. It is possible that the complex muscular acts which we group under the head of grasping are operated in similar fashion. For example, the act of writing is essentially a modified grasping movement. When we are writing we think not about the muscular movements but about the sense of what is being written, and it may be that the actual control of the muscles in this case is something like that which we know exists in locomotion, although this is by no means certain. It is of course a very great advantage to us to have these complicated activities, which we are performing so constantly, carried on as reflexes without conscious effort on our part. If we had to think about the movement of every muscle, whenever we started to walk, our minds would be so taken up with the act itself that there would be no freedom for anything else; but since the reflex machinery takes care of these muscular movements for us, our minds are left free to deal with other matters at the same time as the muscular actions are going on.

Not only is the simple act of walking carried on for us without mental effort on our part, but to a very large extent the guidance through the sense of sight becomes reflex also. Thus, as we go along the street, we step down from or up on to the curb at the crossings without noticing particularly what we are doing. The approach of the curb is registered through the eyes, and the sensory disturbances thus set up are carried into the brain and intermingled with those from the organs of muscle sense and equilibrium, so that the motions are adjusted to carry us safely down or up. This reflex guidance through the eye becomes so perfect that one who habitually walks over a certain course, as a man in going from his home to his place of business, ordinarily goes the entire distance with scarcely any conscious attention to what he is doing; in fact, persons who move from one locality to another frequently, when they start to walk home, unless they pay attention, find themselves going to the old home instead of the new. The reflex machinery which guided them to their old home works so well that only by conscious interference with it are they able to direct themselves to the new one. Whenever an act of locomotion is learned, it is carried on at first by nervous disturbances from the cerebrum, just as are muscular actions of all sorts; but, as time goes on, there seems to be a transference of the control from the cerebrum to the cerebellum, and as this happens the act is more accurately performed. We have an amusing illustration of this in persons who are learning to ride a bicycle. So long as they are making movements of the handle bars and at the same time pedaling by pure acts of the will, they proceed with the very greatest difficulty; after these actions become habitual, they are carried on easily and skillfully. The great effort of athletic trainers is to develop what is called “form.” What this really means is that they try to teach the athlete the best way of making the desired motions, and then drill him in that particular way, so that it becomes completely reflex with him. The more fully this is done, the better will the athlete perform.

All that has been said about the nervous system thus far has applied to its duty of controlling the activities of skeletal muscles; namely, the muscles by which the body carries on its principal motions. These, as already stated in Chapter VII, are not the only kind of muscles in the body. There are also the smooth muscles and heart muscle. There is also another kind of functional metabolism besides the metabolism of muscular motion over which the nerves have control. This is the metabolism of secretion, or the manufacture of special chemical substances by gland cells. In later sections of the book we shall hear more about what the glands do, but here we have to say something about the way they are controlled through the nervous system. We speak of them along with the smooth muscles, because it happens that the same nervous arrangements are found in both cases; the smooth muscles and the glands have a precisely similar kind of nervous control. In appearance the nerves which lead from the central nervous system to the smooth muscles and the glands do not differ much from other nerves. They are composed of nerve cells, just as are all the other nerve pathways. It happens to be true that the sheaths which inclose the axons of these particular nerves usually have little or no white material in them, so that nerve trunks which are made up of these fibers look gray instead of white, as do the other nerves. Just why there should be this difference, we do not know. Of more importance than this difference in appearance is the fact that the connection from the central nervous system to the smooth muscle cell and the gland cell in no case is made up of a single nerve cell, but always of a chain of two. In the case of nerve paths to skeletal muscles the cell body of the motor nerve cell lies either in the spinal cord or the brain and its axon extends all the way from there directly to the muscle fibers. In the case of smooth muscles and glands a connecting nerve cell extends out from the central nervous system some distance and makes its nerve junction with the motor nerve cell part way between the central nervous system and the muscle or gland. These connecting cells are much branched, as are all connecting cells; also the motor cells which connect with smooth muscles or glands are very much branched; the result is that a single nerve cell starting from the spinal cord finally connects with a large number of smooth muscle cells or gland cells. We do not have, then, in these organs the fine subdivision of control that there is in the skeletal muscles, for a single skeletal motor cell in the spinal cord connects with one or, at the most, a very few muscle fibers. If we consider the uses to which smooth muscles are put, we shall see that finely subdivided control is not important, as it is in the case of the skeletal muscles, where the movements have to be graded very accurately. Smooth muscles are present in the stomach and intestines and other abdominal organs which make movements. They are found also in the walls of the small blood vessels where their contractions or relaxations can change the caliber of the vessels, and so affect the amount of blood passing through them. A good example of this action is seen in the color of the face; when one is pale, it is because the small blood vessels just under the skin of the face are much contracted, and very little blood can pass through; when flushed, on the other hand, the blood vessels are relaxed and there is a large amount of blood passing. These contractions and relaxations of the blood vessels are performed by movements of the smooth muscles. Another interesting place where there are smooth muscles is at the roots of the hairs. Everybody knows that the hair is supposed to stand on end when one is frightened. Most of us have thought of this as a picturesque figure of speech, but as a matter of fact it actually does happen. A human being has such a small equipment of hair that its rising on end does not make much of a showing, but in an angry dog or frightened cat we see it in all its glory. If one looks at the forearm when it is so chilly that “goose-fleshing” occurs, he sees his hair then standing clearly on end. In fact, goose-fleshing is caused by contractions of these smooth muscles just under the skin. Every hair has a few smooth muscle cells at its base and they are attached in such a way that, when they contract, the hair is pulled from the slanting to the erect position. The surface of the skin acts as the fulcrum on which the motion is made. Besides these places where smooth muscle is found there are some in the eye. The muscles of accommodation and also those by which the size of the pupil is changed are smooth muscles. There are some also in the small bronchial passages. In persons who suffer from asthma these smooth muscles become contracted, and the difficulty in breathing experienced is the result of the partial closing of the air passages caused by this contraction. Glands which are under the control of the same kind of nerves as control the smooth muscles are found in many places in the body; in the head we have the tear glands and the salivary glands; in the trunk there are the gastric glands, the pancreatic, and various others about which we shall hear later. With the single exception of the muscles of accommodation there is no case either among the smooth muscles or the glands where there is any occasion for a finely subdivided control. When the muscles of the stomach or intestines become active, a great many fibers work together. When the hair stands on end, a great many hairs become affected at once. A flushing or paling of the skin is also a matter of a large surface. The same is true of gland secretion. In an active gland there is no picking out of a few cells to do the secretion, but a large part of the gland, if not the whole of it, becomes active, whenever any of it is. The advantage of the construction of the nerve paths from the spinal cord to the smooth muscles or to the glands is that a single nervous disturbance from the central nervous system can affect many muscle or gland cells all at once. In the case of the skeletal muscles, on the other hand, the only way a great many fibers can be made active at once, is by sending out nervous disturbances through a great many nerve cells. There is undoubted economy in having the control of the smooth muscles and glands carried on by means of comparatively few discharges from the central nervous system.

A peculiarity of the structures that we are talking about now is that we have very little voluntary control over them. This contrasts sharply with the completeness of our control over the activities of our skeletal muscles. With very few exceptions we can start, stop, or regulate skeletal muscle activities at pleasure, but the activities of our smooth muscles and of our glands are not under our control in the same way. We cannot cause ourselves to turn pale by an act of the will, nor can we set the stomach to working when we wish. This latter inability is a real misfortune, for there are many cases of acute indigestion that are due to stagnation of the stomach, and if we could set the stomach into operation at will, these would be avoided. Since we have not this voluntary control, it is of the utmost importance for us to learn how the control is really carried on, so that the motions necessary to health can be induced in a normal manner.

To a very large extent the smooth muscles and glands are subject to reflex control; that is, nervous disturbances coming in from the sense organs pass over by way of connecting nerve cells to the nerve pathways leading out to the smooth muscles or to the glands. Examples of this kind of control are seen in the paling of the skin, when one is chilly, or its flushing, when one is warm; in the copious secretion of saliva into the mouth that one can get by chewing a rubber band; or in the secretion of tears that comes when the front of the eye is hit.

A second kind of control of the smooth muscles and glands is their control through the emotions. We have already given illustrations of this in the paling of the skin when one is frightened, or the erection of the hairs in the tail of an angry cat. For a long time it has been known that most of our emotions, if not all of them, are accompanied by some sort of bodily change; laughter and crying are familiar illustrations. To a certain extent, as in these instances, skeletal muscles share in the bodily effects of emotions, but their most important examples are found in the smooth muscles and glands. Every serious display of emotion is accompanied by a discharge of nervous disturbances into some of the smooth muscles or some of the glands. This fact has long been known, but its meaning has become clear only very recently. Before attempting to explain this, an additional word will be necessary about the nervous control of smooth muscles. These muscles differ from skeletal muscles in one important particular, which is this: when a skeletal muscle is aroused to contraction, it will relax as soon as the stream of nervous disturbances stops. To keep it in contraction a continuous stream must come into it through the nervous system. This is not true of smooth muscle. When a smooth muscle has been caused to contract, it is very likely to stay contracted without further action on the part of the nervous system. In fact, in very many cases another kind of nervous disturbance must be sent into it in order to make it relax. This means that the smooth muscles, or at least most of them, have two sets of nerves coming to them; one set which causes them to contract, the other set which causes them to relax. Both these sets of nerves are of the type described above as supplying the smooth muscles and glands. A good illustration of the double action is found in the stomach. There is one set of nerves whose activity causes the walls of the stomach to perform the churning motions which go on ordinarily during digestion. There is another set of nerves whose effect upon the stomach is to cause all motions in it to stop. This same double arrangement of nerves is found in many of our most important smooth muscle organs. We said a moment ago that every emotional display is accompanied by an outflow of nervous disturbances to the smooth muscle or gland structures. We now see that what happens in the body is determined by which of the possible pathways is followed by the nervous disturbance. One turns pale, when frightened, because in the emotion of fright the nervous discharge is over the nerves that cause the blood vessels of the face to contract. The mouth waters at the sight of delicious food, because the pleasurable emotions aroused thereby cause the nerves to the salivary glands to become active. As was said a moment ago, for a long time there seemed to be no rhyme or reason about these emotional reactions. Why, for example, should worry cause the muscles of the stomach to stop completely, thus bringing on thousands of attacks of acute indigestion? We now have a clue to the meaning of these facts, which is about as follows: The emotions are to be looked upon as part of the machinery by which bodily well-being is promoted. Emotions of pleasure signify that matters are progressing favorably, and the nervous discharges accompanying these emotions are those in accord with well-being. For example, the activities of the alimentary tract in digestion are of a kind that prosper when the body as a whole is well off. Disagreeable emotions, on the other hand, are associated with the necessity for self-preservation. Thus anger or fear arise in the presence of an emergency calling either for combat or flight. The nervous discharges that accompany these emotions are found on analysis to bring about bodily changes whose effect is to prepare the body successfully for one or the other. This is best shown by actually naming over some of the bodily changes that occur. We may begin with the top of the head and work down. The hair stands on end; this is of no importance in human beings, but in many animals contributes materially to the ferocity of the appearance. The pupils are dilated, aiding vision; the face is pale, and the mouth is dry; not because these in themselves are of importance but because they represent a diversion of energy from unimportant to important parts of the body. The blood that ordinarily flows through the face is shunted out of it into the brain and muscles, where it will really help in the emergency. The activity of the salivary glands is stopped to free the energy that they would consume for use in the brain or muscles. The heart beat is quickened; this again, by making the circulation more efficient, helps the animal to fight effectively. The activity of the stomach is suspended for the same reason as is that of the salivary glands. What we have is a marshaling of the bodily forces into the organs which are of immediate importance in meeting the emergency; namely, the nervous system and the muscles. The other parts of the body, which are necessary in the long run, are allowed to stop functioning for the moment. This behavior of the body in an emergency illustrates better than anything else could the point that our muscles, nerves, and sense organs make up the really important parts of us and the rest of the body has simply the duty of keeping these in good order. In time of emergency all the energies are devoted to the task of keeping these parts of us at highest efficiency, however bad may be the effect on the other less necessary organs; so it is that one who worries himself into an attack of indigestion is really preparing for an emergency by sacrificing the less essential for the more essential activities. The practical difficulty in this is that the worry almost always fails to be of use, either because the expected emergency does not arise at all or else arises so far in the future that the immediate bodily changes caused by the worry do no good. Evidently worry is an emotion to be combated. The old adage about not crossing the bridge until it is reached has really a great deal more to do with the preservation of health than most of us realize.