CHAPTER XIX

KEEPING WARM IN WINTER AND COOL IN SUMMER

A good deal of what has been said thus far in the book applies to nearly all kinds of animals about as well as it does to man. We have now to take up a feature found only in two great groups of the animal kingdom--birds, and the four-legged animals, to which are given the name of mammals. This peculiarity is commonly called warm-bloodedness. What we really mean when we say that an animal is warm-blooded is that the temperature of its body runs about the same summer and winter, day in and day out. An animal that we call cold-blooded, on the other hand, cools down when in a cold place, but is warm when in a warm place. It therefore has a very variable temperature as compared with the almost constant temperature of warm-blooded animals. The maintaining of the warm-blooded condition, which means really the maintaining of a constant temperature, involves only one thing, namely that the amount of heat lost from the body shall exactly balance the amount of heat produced in it. If more heat is produced than is lost, the temperature must rise; if more heat is lost than is produced, the temperature must fall. These are simple facts of physics which apply as well to the body of an animal as to any other source of heat.

All animals produce heat, because in all of them metabolism is going on, which means that in all of them energy is being liberated, and it is one of the fundamental laws of physics that whenever any energy is liberated all of it which does not take some other definite form is certain to appear as heat. The other possible forms that the energy liberation of the body may take are chemical energy in the manufacture of various materials and the energy of motion. Whenever either of these processes takes place a large amount of heat is produced in connection; this means, from the standpoint of mechanics, as we have already seen, that the body is an inefficient machine; if it were perfectly efficient, the energy that it liberates might all go into the form either of motion or of the manufacture of materials, with none left over to appear as heat; but since it is only 20 per cent efficient, four-fifths of all the energy that is displayed actually takes the form of heat. While this is a marked inefficiency from the mechanical standpoint, from the standpoint of bodily well-being it is by no means a bad thing, since it is this constant production of heat which makes possible in birds and mammals the maintenance of a constant and rather high temperature. That it is a great advantage to the body to be warm all of the time is clear when we compare the possibilities of our lives with those of cold-blooded animals, like insects or frogs. Whenever the weather cools down they necessarily become inactive since, as we saw in the third chapter, when protoplasm is cooled, its metabolism necessarily slows down, and when the cooling reaches a certain point the protoplasm becomes completely inactive. The temperature of 98½ degrees F., which our bodies maintain, is a temperature which is very well suited for an active metabolism; by keeping this temperature all the year around they are able to show this metabolism all the time instead of only in summer, as in the case of the cold-blooded animals.

As we said a moment ago, the maintaining of a constant temperature is altogether a matter of making the loss of heat balance the production of heat; the production of heat is altogether a matter of the metabolism; practically the whole of the resting metabolism takes the form of heat, since apparently the expenditure of energy in keeping the protoplasm alive is extremely wasteful; nearly all the energy that is actually liberated takes the form of heat, almost none of it being used in actual manufacture of material. This is, as has already been said, a very constant metabolism; the functional metabolism on the other hand is extremely variable depending on how hard we use our muscles; the total amount of heat produced from hour to hour is scarcely ever the same except when we are asleep; this means that the loss of heat, which has to balance the production of heat, must vary from hour to hour exactly parallel with the latter. In birds and in all mammals, except man, the adjustment of heat loss to heat production is almost wholly automatic; the animal or bird does very little to control it. We do see, however, a few examples of effort on the part of animals either to prevent heat from being lost too rapidly or to favor its more rapid loss. Thus, on a cold day, a cat or dog is apt to lie curled up or with its legs bunched under it; on a very warm day, on the other hand, it will lie stretched out as much as possible, and in the case of the dog, with the tongue hanging out full length. Birds sometimes are seen with feathers ruffled up apparently in an effort to keep warm. Both birds and mammals seek sheltered places in which to sleep, where they will be as little exposed as possible to cold winds or rains. In man the automatic adjustment of heat loss to heat production is very imperfect, in fact, it would not enable men to live naked except in the tropics. It is true that outside the arctic regions men might go naked and still make the heat production equal the heat loss during the waking hours by very vigorous exercise, but during the hours of sleep the loss of heat would be certain to be much more rapid than its production and so the body temperature would fall in severe weather enough to cause death. Man maintains himself outside the tropics, then, by using artificial aids for maintaining the balance between heat production and heat loss; these are of three sorts; first the use of clothing, second the use of shelter, and third the use of artificial heat. It is really a very curious fact when one stops to think of it that, although many animals enjoy artificial heat and gladly bask in it when opportunity offers, no animal has ever discovered the simple fact that throwing sticks on a fire will keep it going, and so no animals, except man, have ever made any real use of artificial heat. When we think that the progress of human civilization has been accomplished wholly in parts of the earth where clothing, shelter, and fire are necessary to human existence, we realize that these, instead of being mere incidents to our life, lie really at the very basis of advancement. Hairless man is evidently a tropical animal; if he had not devised means of maintaining himself outside the tropics, there is no reason to suppose that he would have behaved any differently than have the savages that inhabit those regions at the present time.

Clothing, shelter, and fire all operate to prevent us from losing heat too rapidly and so have their great value when the weather is cool; we also can and do bring about an increased heat production in cold weather partly by exercising more actively and partly by increasing the amount of protein in the diet, and so bringing into play the stimulating effect of that substance on metabolism. This latter is seen perhaps most strikingly in the experience of the Eskimos; when one of them comes in from a hunting trip in the depth of winter very much chilled, he scarcely stops to warm himself at all, but with the greatest haste gulps huge quantities of meat that are barely thawed out, not really cooked in our sense of the word; in fact, frequently the meat that is eaten is so cold that it makes the Eskimo chillier than before; as soon, however, as digestion and absorption have commenced, so that the stimulation of metabolism can begin, the rapid production of heat in the body warms it up to the point where complete comfort is obtained.

We have no other means of increasing heat production in our bodies except by muscular exercise or eating protein. It is interesting to note, however, that there is a form of involuntary muscular exercise which comes into play when we need to produce more heat in order to balance too rapid loss; this involuntary muscular action is the familiar shivering which one does when chilly. It is reflex in the strict sense, that is to say, the stimulation of cold on the skin arouses nervous disturbances which pass to the muscles in various parts of the body and set them into the violent movements which we call shivering. The functional metabolism of shivering is much greater than we might at first suppose, and the production of heat is correspondingly rapid. In fact, if one who feels inclined to shiver encourages it instead of attempting to keep from doing it, he will very quickly produce enough heat within his body so that he will no longer have a disposition to shiver.

We have seen above the various ways in which heat is produced in the body, and the artificial means we employ to prevent heat from being lost too rapidly. It is clear that all of these make up what we may call coarse adjustments. They tend on the whole to cause heat to be produced more rapidly when more is needed, or to prevent too rapid loss when conditions are of a sort to bring it about. Nothing that has yet been said accounts for the fine adjustment, that is, for the actual maintaining from moment to moment of a balance of the heat loss with the heat production. This fine adjustment is purely automatic and is carried on in us by two distinct means. The first of these is by an automatic regulation of the amount of blood flowing through the skin and so, within certain limits, of the temperature of the skin. It is evident that if the skin is warm heat will be given off from it more rapidly than if it is cool; a condition in which the balance is being upset by the failure of the body to lose heat rapidly enough can be corrected, at least in part, by causing the skin to become warmer and so more heat to be given off. The machinery for regulating the amount of blood flowing through the skin is the vasomotor system, about which a good deal has already been said. This system operates reflexly, stimulation of cold on the skin tends to diminish the flow of blood through it by contracting the blood vessels, while warmth on the skin has just the opposite effect, causing the blood vessels to become flushed and the blood flow to be more rapid. It is true that there is a reddening of exposed parts of the skin in extreme cold, as we all see frequently on our nose or ears, but this is a purely local effect and does not mean that the blood is flowing through the region rapidly enough to keep up its temperature and so favor the loss of heat. These changes in the amount of blood in the skin are very effective in the fine regulation of heat loss. The body can get rid of heat very much more effectively when the skin is flushed than when it is pale. There are, however, limits to the usefulness of this arrangement. Even when the skin is as pale as it can become it still is warm, and on a cold day it continues to lose heat more rapidly than is desirable. It is to prevent this that clothing is worn. Clothing operates for people as fur does for animals. It establishes a nonconducting layer between the skin and the outside. This nonconducting layer warms up to the temperature of the body and so hinders the escape of heat. The actual nonconducting material is the air which is caught in the meshes of the fabric, or in the case of fur-bearing animals among the different strands of the fur. The effectiveness of clothing for conserving heat depends altogether on the degree to which it imprisons air in its meshes. In this respect wool is the most effective of all fabrics, cotton next, silk and linen having very little effectiveness. This explains why wool is preferred for winter clothing, linen and silk for summer.

The temperature of the body as a whole is, as we have seen, about 98½ degrees F.; this figure represents the maximum temperature that the skin can reach, even when it is flushed to its utmost. In the warm weather of the summertime the temperature of the air frequently mounts to or even above that point and it is evident that under such circumstances the body cannot give heat directly from itself to the surrounding air, and no amount of flushing of the skin will enable it to do so. It is under these circumstances that the second method of getting rid of heat comes into play, namely, the evaporation of sweat. We have spoken of the sweat glands in a previous chapter, but have left their action and the description of the skin in which they lie for this point.

The skin serves a number of purposes; it is the great protective layer for all the parts beneath; in order that it may serve for this it must itself be reasonably free from injury. This is secured by having the outer layers of the skin composed of a horny dead material. Just beneath this dead layer is a layer of living cells tightly packed together and having exposure to the body fluids only at their under side. Cell multiplication goes on in these cells continuously and rapidly; as the cells divide, they grow, the result being that those that are underneath are constantly crowding those above them farther and farther out toward the surface; this cuts them off from their food supplies and so they die. This layer of dead cells becomes very closely packed and makes up what is commonly called the horny layer of the skin or the “cuticle.” The outer part of this cuticle is constantly rubbing off and, of course, as this happens any dirt that may be clinging is rubbed off as well. A large factor of cleanliness is the constant rubbing off from the surface of the body of its outermost layer. The extent to which this happens can be appreciated by anyone who has ever had a broken bone and has had to have an arm or leg put up in a splint or cast for several weeks. At the end of that time there is a great accumulation of dead skin waiting to be rubbed off.

Bathing is a very efficient aid to cleanliness; when baths are for this purpose, warm or hot water, with an abundance of soap, should be used; the choice of soap is an individual matter determined largely by the sensitiveness of the skin. Chemically there is no very great difference between one kind of soap and another. Some of them contain materials which are more irritating than those found in others and are to be avoided by persons whose skins are easily irritated. The common belief that a bath should not be taken immediately after eating rests on the feeling that the flushing of the skin that results from the contact with the warm water may divert blood unduly from the alimentary tract and so interfere with the process of digestion. This may very well be true if a very hot bath is taken; where the bath is only a degree or two above body temperature little trouble is to be feared from this cause. A cold bath shortly after eating sometimes gives rise to cramps either in the stomach or in the muscles. Not much is known about these cramps or about the causes which bring them about. In connection with its use as the protective surface of the body the skin has scattered over it numerous glands known as the _sebaceous glands_, which secrete an oily substance that is spread more or less completely over the skin. This oil probably helps to keep the skin soft and also to some degree to make the skin waterproof. One of the reasons why soap has to be used in bathing is to dissolve off this thin film of oil which is ordinarily over the surface of the body.

Another use of the skin is as a great sense organ; as we have already seen, the structures for touch, for temperature, and for pain are present in the deeper layers of the skin. These consist of tiny cell masses into which the tips of the sensory nerves pass. In the case of the nerves of pain there are believed to be no special end organs, but the nerves end nakedly among the cells of the skin.

The third use of the skin, and that with which we are particularly concerned here, is as the fine regulator of body temperature. We have already seen in part how the skin works in this connection, through being warmer when there is more blood flowing through it and cooler when there is less. Its other heat-regulating mechanism is made up of the _sweat glands_. These, as the name implies, are tiny glands in the skin; they consist of tubes opening on the surface and at their inner end coiled up into a sort of knot. In this knot are a great many fine blood vessels, so that each sweat gland has an abundant blood supply. So far as we can tell, the sweat glands act as filters, corresponding in that respect to the filtering tips of the kidney tubes. One interesting thing about them is that the filtering action is controlled by nerves. These nerves belong to the system which controls the smooth muscles and glands; when the nerves become active the sweat glands are also active, and there is rapid filtration of water and inorganic salts from the blood into the sweat glands and out to the outside of the body. Small amounts of organic materials come out also in the sweat, and these are responsible for its characteristic odor. They evidently differ in different people, since all dogs and some persons are able to recognize individuals by their odor, and body odor is largely the odor of sweat.

The sweat glands are carrying on their filtering activity most if not all of the time; we are not usually aware of it because the sweat comes out in exceedingly fine drops which evaporate as fast as they are formed; it is only when the sweat is poured out faster than it evaporates that we become aware of its presence. If the air is very full of moisture, sweat will stand on our bodies even though it is not being formed more abundantly than usual. This is because of its inability to evaporate into air already laden with moisture. Usually, however, when sweat appears on the surface of the body it is because the sweat glands have become more active. A couple of paragraphs ago we saw that the sweat glands are under the control of nerves; these arise like the other nerves that control smooth muscles and glands from a center in the brain stem; this particular one is called the sweat center. It may become active through various influences. The one most commonly arousing it is a rise in the temperature of the blood. Whenever we begin to exercise, heat is produced very rapidly, warming up the tissues in which the activity is going on and, since the blood is flowing through them rapidly, warming the blood up as well. This warm blood circulates all through the body, tending to raise all parts of it to the temperature of the active regions, or, as it passes through the skin, to cool by loss of heat from the surface. When this warm blood enters the brain stem it arouses the sweat center and an increased secretion of sweat results. The importance of this is that the evaporation of water, no matter at what temperature, requires a large amount of heat. This heat is abstracted from the nearest place, which in this case would be the body itself, so that the evaporation of sweat acts powerfully to cool off the body. In hot weather this is really our only effective means of getting rid of heat, for if the body is no warmer than its surroundings it cannot lose heat directly, but sweat can evaporate, taking heat with it, no matter how warm the surroundings may be. We said a moment ago that a high percentage of water in the air would hamper the evaporation of sweat. The practical workings of this fact are seen in connection with heat prostrations in the summertime. Careful scrutiny of the weather reports will show that these are much more numerous as well as more severe on days of high relative humidity than on days that are simply hot, but without much moisture in the air. As the sweat evaporates from the body, the resulting water vapor has a tendency to linger in the neighborhood and so to interfere with further evaporation. Under these circumstances it is desirable that this moisture-laden air be moved away and fresh air brought into its place. This is the benefit we obtain from fanning. The air that is brought upon us by a fan feels cooler than it really is; this we realize when we notice how comfortable the draft of a fan is even on the very hottest days. When one fans oneself in the old-fashioned way, the heat produced in working the fan is often almost enough to balance the loss through the improved evaporation of sweat. An electric fan, on the other hand, is a very powerful aid to comfort in hot weather. In fact it is hard to think of any mechanical device that has contributed more in recent years. An amusing illustration of the importance of moisture in the air in relation to bodily comfort is in the case of the man who thought he would obtain a cool-weather office in hot weather by installing a block of ice and causing his electric fan to play upon it. Very much to his surprise and quite contrary to his expectations the room quickly became unbearably sultry. The rapid evaporation of the ice filled the air so full of water vapor that evaporation of sweat could not take place and the persons in the room were much worse off than if they had contented themselves with merely keeping the warm and comparatively dry air of the room in rapid motion. Although the evaporation of sweat is a very effective means of getting rid of heat from the body it does not work perfectly in times of great muscular exertion in very hot weather. Careful studies of men in steel industries and other places where hard work is done in intense heat have shown that the production of heat during the course of the working day slightly out-strips its loss, so that by the end of the day it is the regular thing for the body temperature to be up two or three degrees, to 100 or 101 degrees Fahrenheit. Upon finishing the work the temperature quickly falls to normal, so that there is no reason at present for supposing this daily rise to be seriously harmful.

In fever the production of heat becomes greater than its loss for a time and so the temperature goes up. It must not be forgotten that presently the loss again balances the production; if it did not, the temperature would keep on rising until death resulted; usually the greater loss of heat from the body at the fever temperature is sufficient to restore the equilibrium, but it will be clear that the temperature cannot fall again until the heat loss exceeds the heat production. There has been a good deal of discussion as to whether fevers are caused by an increase of heat production or by an impairment of the machinery for getting rid of heat. There is no doubt that in fevers the resting metabolism is much greater than in health; it is not by any means always the case, however, that the total metabolism of an individual lying quietly in bed with a fever is greater than that of a healthy individual doing heavy manual labor. We must therefore look to an interference with the process of losing heat to account for the rise in temperature. The distribution of blood through the skin in fever does not differ from that at other times when the body is warm; it is the usual thing in fevers for the skin to be much flushed. There is, on the other hand, a definite impairment of the sweat-secreting mechanism. It is one of the most familiar facts about fever that the skin is not only hot but very dry. It is also well known that by inducing sweating the temperature can often be brought down. It is evident that the poisons which are responsible for the fever have at least two effects; to increase general resting metabolism to a high level and at the same time to interfere with the ordinary regulation of the sweat glands, so that, even though the temperature of the blood is several degrees above normal, the sweat glands are not stimulated to active production of sweat.

A fever is very uncomfortable to the person suffering from it, but is not in itself particularly dangerous, unless it mounts to a very high degree. The body wastes away rapidly, of course, because of the increased metabolism which is usually not balanced by any taking of food. Theoretically fever patients should eat as abundantly as laborers; practically this is impossible of accomplishment in most cases because of the depressed condition of mind and body of the patient. In some particularly wasting diseases, like typhoid, feeding has been carried on with some degree of success. The old practice with regard to fevers was to restore the patient to comfort as quickly as possible by getting rid of the fever, that is, by lowering his temperature to normal; at the present time the tendency in medicine is not so much to strive with the result of the poisoning as to seek to rid the body as quickly as possible of the poison and of its source. For that reason modern physicians are much less concerned with the degree of fever shown by the patient than with the extent to which the poisons in his body are being controlled or are becoming masters.

In all our discussion of the regulation of body temperature it is important to remember that our feelings of cold or of warmth depend altogether on the temperature of the outermost one-quarter inch of our body surface. No matter how cold or how warm we may feel, as soon as we get to that depth below the surface, we find the body temperature the same, namely 98½ degrees. We feel cold when the skin is cool; the skin is cool because not much blood is flowing through it; the stimulation of the surrounding cold on the skin has aroused reflex vasoconstriction and so diverted the blood out of the skin into other parts of the body. This means that the mechanism for preventing loss of heat is working as effectively as it can and, therefore, that the heat which the body produces is being conserved. Experience shows that in reality the feeling of cold is very largely a matter of the condition of the extremities. If the hands and feet are warm, one rarely finds the cold uncomfortable; on the other hand, cold extremities produce a very general feeling of discomfort, even though the rest of the body may be warm. The lesson to be drawn from this is to pay particular attention to keeping the extremities warm. Where workmen are standing on a floor it is much more important that the floor be warm than that the air of the room as a whole be so. Of course, it must not be cold enough to allow the fingers to be chilled and stiff where manual work is being done, but it has been found that a considerably lower factory temperature is endured when the floors are warm than when the floors are cold.

We feel cold in cold weather because the body is conserving heat; in warm weather, on the other hand, we feel warm because the skin is flushed and the body is losing heat as rapidly as it can. The practical bearing of this is that it might be disastrous to feel particularly warm in cold weather. Not, of course, if the feeling of warmth be due to a very rapid production of heat, as in vigorous exercise, but if it is due simply to a flushing of the skin when not much heat is being produced, the body is being deprived of what heat it has and a condition which cannot long be endured in cold weather will result. It happens that exactly this situation is met with in connection with the use of alcohol. One of the important effects of alcohol is to produce a flushing of the skin; this is a direct drug effect and has nothing whatever to do with the amount of heat that is being produced in the body. The result is that one who, by a few drinks, fortifies himself, when starting out for a long cold ride, feels very warm and comfortable as long as the alcoholic effect persists; but as soon as this passes off he is very much colder than he otherwise would be, since during all this time he has been losing heat rapidly instead of conserving it. It used formerly to be not at all uncommon in the cold parts of the country for persons to perish from exposure on long drives because they had attempted to keep themselves warm by taking alcohol, when if they had conserved their bodily heat instead of wasting it for the sake of comfort, they would probably have reached their destinations in safety.

While we are on the topic of temperature regulation and the part played by the skin generally in bodily activities a word should be said about bathing for other purposes than cleanliness. There is a good deal of discussion about whether the morning cold bath is really a health measure or is simply a fad; there can be very little doubt that persons who react well to the cold bath find that it contributes definitely to the enjoyment of life. It is particularly true that after a night of disturbed rest the morning cold bath makes one feel much more ready for the duties of the day, whether he actually is so or not. The value seems really to be in what is known as the _reaction_ in which the first chill is followed by a warm glow. The bath should under no circumstances be prolonged beyond the time when a good reaction is obtained. Brisk rubbing with a harsh towel helps the oncoming of the reaction very much. For those who would like to take a cold bath but have found it too much of a hardship a hint may be helpful, namely, that if one enters the bath with the feet warm there is practically no discomfort connected with taking it. To step into a cold bath when the feet are already cold causes them to begin aching almost instantly and usually nullifies all the benefits the bath affords. One should either enter the bath immediately after rising, before there has been any time for the body to become chilly, or if one prefers to shave or carry on other activities first, it is a good plan to give the feet a momentary preliminary bath in hot water. When this condition is fulfilled, the cold bath will be both enjoyable and beneficial to the majority of people. Very hot baths have a distinct relaxing effect; they favor the onset of sleep and can be taken to best advantage just before retiring. There is nothing in the common idea that a hot bath opens the pores of the skin; the only pores the skin has are the openings of the sweat glands, and these do not vary in size. What a hot bath does do, is to cause a marked flushing of the skin as the result of which the body loses heat rapidly. It is necessary, therefore, after a hot bath to wrap oneself warmly so that excessive loss of heat shall not occur.

In finishing this topic, the subject of catching cold must be brought up, together with its relation to the regulation of body temperature. There is no doubt at the present time that common colds result from infection; in that sense they are acute diseases. It seems to be the fact, however, that the organisms which cause them are pretty constantly present in the air passages of our throats and lungs ready to invade the body whenever opportunity offers. Very recently it has been shown that rapid chilling of the skin is accompanied by contraction of the blood vessels in the linings of the throat and air passages. The effect of thus cutting down the blood supply to these linings seems to be to make them more ready of access by the organisms of colds. At any rate the fact that when the skin is suddenly chilled a cold frequently develops is best explained on this basis. Since colds are infectious, the organisms causing them may sometimes succeed in penetrating the linings of the air passages even when no change in the latter has occurred. It may therefore happen, and frequently does, that one develops a cold without being able to refer it to any time when there was a sudden chilling of the skin. Also it can easily happen that the chilling may not lead to a cold, since if the organisms do not happen to be abundant or if the general bodily resistance is high, the infection will not get a foothold, even though the linings of the air passages would permit the organisms to enter. One’s best precaution against colds would naturally be to avoid having the organisms which produce them present in the throat and lungs. Since this usually cannot be done, the remaining practical measures are to keep the general bodily health as good as possible and to avoid conditions which lead to a sudden rapid chilling, such as sitting in a cold draft when heated.