WE have talked a good deal about muscles and the different sorts of activities they can perform. We have also mentioned the fact that the skeletal muscles are under accurate nervous control. Our next task is to investigate the control of this nervous control; in other words to find out just what it is that causes the nerves to stimulate the muscles so that they shall perform as skillfully and usefully as they do. In Chapter II we saw that our bodily movements are adjusted to our needs through the sense organs. These bring information of the situation and we act accordingly. We may group the kinds of information which the sense organs furnish under three heads; first, what is going on inside our bodies; second, what is happening at the surface of the body, and third, what is happening at a distance from us. The senses which bring the first kind of information are called the internal senses; the second group are the contact senses; and the third are the distance senses. We need to remember that the primary purpose of the senses is to guide our muscles, and that our muscles are to find food for us, to keep us from bodily harm, and to assist in the perpetuation of life by propagating and caring for the young. By keeping these facts in mind we shall have no difficulty in understanding the way in which the various senses do their work.

Pain, hunger, and thirst are the internal senses with which we are most familiar. Pain is evidently a protective sense. It is never aroused unless something is amiss; for that reason pain should never be neglected. Of course, in the majority of cases the pain is due to some simple disturbance which can be located, and if no permanent harm is to follow, or if no relief is possible, the heroic bearing of the pain is meritorious; but thousands of women, thinking mistakenly that to complain of suffering is a sign of weakness, or hoping to spare loved ones distress, bear in secret or make light of pains that are the signs of insidious disease, curable if taken in hand early enough, but sure to cause acutest suffering and untimely death if allowed to go on unchecked. Unfortunately our most dangerous internal enemies, the organisms of infectious disease, do not give warning of their attack by causing pain until the disease itself is so far advanced that there is no escaping it. In this respect pain falls short of being efficient as a means of warning us against impending injury.

Hunger and thirst are the stimuli which drive us to the taking of food and water. It is interesting to think that of all the living things that roam the earth only men have discovered the connection between the taking of food and the avoidance of starvation; all other animals are impelled to nourish themselves wholly through the operation of these senses. There are two distinct phases to hunger. The first is appetite, and this by itself seems not to be a sense in the strict meaning of the word, but rather a memory of agreeable experiences associated with the taking of food. In man appetite is often sufficient by itself to lead to eating, as is proved by the frequency with which food is eaten between meals when there cannot possibly be any genuine hunger, but probably in animals it acts to arouse genuine hunger, rather than to cause eating by itself. Genuine hunger is a sense as definite as any other. It is aroused by spasmodic contractions of the stomach. These contractions cannot occur except when the wall of the stomach is in a certain state of tension. Various things can influence the coming on of this degree of tension in the stomach, and so the possibility of hunger. Appetite itself probably does this very effectively. Habit seems also to have something to do with it. Hunger is usually felt just as mealtime draws near, and it is often much keener at noon or night than before breakfast, although the stomach has been longer empty at breakfast than at any other meal. A curious fact about hunger is that it may disappear completely after a few days of complete starvation. Contrary to the popular idea that hunger becomes more and more acute as starvation continues, the testimony of practically all persons who have starved for more than a few days is that all sensations of hunger, as well as all strong longings for food, subside and do not return. This is especially true if the body is kept quiet and if the mind is diverted, so that recollection of meals particularly enjoyed shall not come up.

Thirst is due to actual drying of the throat. When the cells lining that region become deficient in moisture the sense is aroused. The drying may occur from without or from within. When it occurs from without, as in sleeping with the mouth open, relief can usually be obtained by merely swallowing saliva copiously. The same treatment helps for the moment when the lack of moisture is due to deficiency in the amount in the body, but in this latter case no permanent relief can be had except by the taking of water. When the amount in the body falls below the proper level no comfort can be had until the loss has been made good. An interesting thing about thirst is that it is the only sense which is said never to be lost or seriously impaired by disease.

In addition to these familiar internal senses we have some that are less well known. They are for the purpose of what may be described as the routine guidance of the muscles. The act of walking, as we well know, is made up of a series of muscular movements which are both accurately timed and accurately graded. We obtain startling realization of this when we come to the bottom step on our way down stairs without noticing that we have arrived there. This timing and grading are done for us by our bodies without our having to attend to it. The amount of labor that is saved is shown by walking upon railroad ties. These are irregularly spaced, and on that account it is necessary for us to pay attention to every step. There is no comparison between the fatigue of this kind of walking and ordinary progress along a smooth path. The senses that keep track of the position of the body and of individual muscles are known respectively as the equilibrium sense and the muscle-and-joint sense. The equilibrium sense has as its organ a part of the internal ear. Deeply imbedded in the bone is a series of chambers and canals lined with a delicate membrane and filled with liquid. The canals, which are three in number in each ear, are semicircular in shape, and accordingly have been named the semicircular canals. One of them is horizontal; the other two are vertical, and the two vertical canals lie at right angles to one another. This arrangement makes it inevitable that any movement of the head, in any direction whatsoever, will register differently on the canal system than any other movement, which is exactly what is required to make the apparatus efficient as an organ by which motions of the body are kept track of and guided. Along with the semicircular canals is a structure known as the vestibule which registers the position of the head, and so indirectly of the body, when no movements are being made. We are not ordinarily conscious of the working of these senses; they carry on their guidance of muscular movement without our attention. We can, however, pay attention to what they show if we wish. For example, one who is swimming under water is never in doubt as to whether his head is turned up or down, even though his eyes may be shut. His knowledge of position in such a place is obtained from his equilibrium organ, even though he may not be aware of the fact. Sometimes the organ becomes diseased. The results, so far as the victim is concerned, are highly distressing. He usually has to stay in bed because he cannot balance himself well enough to get about.

The organs for muscle-and-joint sense consist of tiny spindles distributed around the joints and embedded within the mass of the muscles. They are arranged so as to be affected by every motion of a joint or every contraction of a muscle. They register not only the fact of motion but also the extent. There is a disease, commonly known as locomotor ataxia, in which the muscle-and-joint sense is impaired or lost, particularly in the legs. The result is that walking becomes difficult and unsteady, and usually impossible when the eyes are shut or the room is dark. This is because the victim learns to make his sight serve instead of his muscle-and-joint sense for guiding his muscular movements, and when this also is withdrawn all knowledge of where his legs are or what they are doing fails, and the only course is to fall down or lie down as quickly as possible.

We have some additional bodily sensations, such as nausea, repletion, fatigue, ill feeling or malaise, which guide our conduct more or less, and are not very different in consciousness from hunger or thirst. So far as is known there are no sense organs by which these sensations are aroused. They are not strictly senses, therefore. We do not know enough about how they originate to say anything more about them.

The contact senses are touch, warmth, cold, and taste. Pain that comes as the result of bodily injury might also be classified as a contact sense, since its cause is something that comes in direct contact with the body from outside, but it differs from internal pain only in its source and not at all in the sensations it arouses, so there is no need of describing it over again. The sense of touch is the fundamental sense; the very lowest animals, even those that have no specially developed sense organs, and few organs of any kind, react to the contact of objects with their bodies just as the highest animals react to the sense of touch. When no other information is available, that of simple contact guides the animal in its securing of food and its avoidance of harm. In accordance with this primitive character of the touch sense, the psychologists tell us that we interpret the information from our more highly developed sense organs, sight particularly, in terms of the feel of objects. When we look at anything our judgment of it actually consists in an idea of how it would feel if we were to take hold of it. Our touch organs consist of tiny spots scattered all over the surface of the body. They are much closer together on some parts than on others. The total number is estimated at a half million or more. A good way to test their sensitiveness is by pressing down on different parts of the skin with fine hairs. When this is done it is found that the most sensitive regions—the tip of the tongue, for instance—are fifty or sixty times as sensitive as the dullest regions, like the small of the back. To obtain sensations of touch it is necessary that there be unaffected points alongside those that are affected. If all are acted on alike, there will be no more sensation than if none is acted upon. This can be shown by dipping the hand into quicksilver. The very heavy liquid presses on all the touch points hard enough to affect them, but since it presses on all alike nothing at all can be felt except along the line where the hand enters the quicksilver where the pressure is strongly marked. It is this feature of the touch sense that makes the wearing of clothing bearable. If we had to feel the contact of the clothes constantly we should presently find them so trying that we could no longer endure them. We do feel rough places and are often seriously annoyed by them, so we can judge what would be the effect if the whole surface were felt as plainly.

Closely related to touch is the sensation of tickling or itching. Curious facts about this sensation are the violence of the feeling that may be aroused by very delicate irritation, drawing a thread along the corner of the nose, for example; the persistence of the feeling beyond the actual irritation; and the effectiveness of scratching as a means of alleviating the condition. Almost nothing is known in explanation of any of these peculiarities.

In addition to organs of touch the skin contains two kinds of organs for perceiving differences of temperature. The first of these detects warmth; the second cold. It is by means of these senses that we judge whether the place where we are is of a suitable temperature in which to remain; whether we should be quiet or active; whether special provisions, like changes in the clothing, are necessary. In the case of both senses the temperature of the skin is the comparison point. We judge that an object is warm or cold according as its temperature is above or below that of the skin which touches it. The ears are usually a few degrees cooler than the hands; thus it is possible for one and the same object to feel cold to the hands and warm to the ears. The two kinds of temperature organs are side by side in the skin, although there are many more “cold” spots than “warmth” spots. Very warm objects affect both kinds, and then we get the sensation that we call “hot,” as distinguished from merely warm. The cold spots are a little nearer the surface of the skin than are the warmth spots; for this reason a hot bath may feel cold at the very instant of stepping into it, although the sensation changes to hot almost at once. We need to remember that our sensations of warmth or cold depend altogether on the state of the skin, and tell us nothing at all about whether our bodies as a whole are warm or cold. Because the blood is always warm a flushed skin always feels warm, and to produce flushing by means of alcohol has long been used as a means of making the body feel warm and comfortable. This may be a serious mistake in cold weather, for to drive the blood to the surface then may mean that the body as a whole will cool off to the point of actual injury. It is better to feel cold and conserve the body’s heat than to feel warm and waste it.

The last of the contact senses is that of taste. This is found only on the tongue. Scattered about on that organ are many tiny sense organs known as taste buds. These are usually in little hollows, so they cannot be affected unless liquids which can enter the hollows are on the surface of the tongue. If the tongue is wiped dry and then dry sugar is sprinkled on it, no sweet taste will develop so long as the dryness continues. The purpose of taste is evidently to give final information about the food after it has passed the inspection of the other senses and has been inserted into the mouth, but before it is swallowed. In the higher animals there has been a subdivision of this sense into two. The other is the sense of smell. In large part smell is a distance sense, and will be treated when we are talking about the distance senses. Smell has monopolized most of the properties of food-judging, so there is left for taste proper only four kinds of perception. These are sweet, sour, salty and bitter. We have, apparently, four kinds of taste buds, one for each of these kinds of taste. All the other sensations that we call taste are flavors, and are really smells. Of the four tastes sour and bitter would probably be called warning and sweet and salty recommending. Only by practice do we come to care for bitter foods, and children usually object just as strongly to those that are sour. Tropical savages, for whom salt is a rarity, esteem it much more highly than sugar, which they can usually get in abundance.

In concluding this chapter we need to remember that the contact senses make up the court of last resort; by the time anything comes close enough to the body to act upon any of them it is so close that the effect in guiding the muscles must be immediate; there is no time for deliberation; whatever the muscles are going to do in response to information thus obtained must be done at once. Later we shall see how this affects our whole bodily make-up.