Physiology: The Science of the Body. Ernest G. Martin

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to make his sight serve instead of his muscle-and-joint sense for guiding his muscular movements, and when this also is withdrawn all knowledge of where his legs are or what they are doing fails, and the only course is to fall down or lie down as quickly as possible.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.