Physiology: The Science of the Body

by Ernest G. Martin

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.

CHAPTER XX
THE PERPETUATION OF THE RACE

WE have tried to take up one by one the chief things that happen in the body, but thus far have emphasized their importance entirely in connection with bodily well-being; that is, we have seen how the body maintains itself against the competition of other living things and in a world full of hazards. Before leaving the subject entirely, a short account must be given of the way in which the race is maintained upon the earth as distinct from the maintenance of individuals. Early in the book we showed that every one of us starts life as a single cell which, by dividing and subdividing, along with continuous growth, finally develops into our large and complicated body. This cell is really the union of two cells; one furnished by each parent; in the body of the parent it formed part of what is known as the germinal tissue, this name being applied because the cell from which we start may be spoken of as the germ of life. As the cell from which the body is to come begins to develop, the cells formed from it quickly become different one from the other, so that very early it can be distinguished that certain cells are to form the future nervous system, others the future muscles, and so on. One of the earliest of these groups of cells to become distinguishable is that which is to become the germinal tissue in the newly formed individual. A great deal of importance has been attached of recent years to this fact that the germinal tissue is set aside almost from the beginning of development, and when we remember that the next generation will be derived from this germinal tissue and from no other tissue, we see that there is a very close relationship of the germinal tissue from generation to generation. The importance of this is in connection with heredity, namely, with the question of the resemblance of the child to the parent. We all know that sometimes children resemble their parents very closely, at other times there is almost no resemblance at all. The main fact of heredity is, as we have just seen, that the child comes from germinal tissue of the parent and not from the parent as a whole. We can think of an individual as a complex body carrying within itself and nourishing an independent group of cells which are to serve as the starting point for the next generation, but with which the individual himself has nothing to do except to provide nourishment. A little later we shall try to show just why we think of the germinal tissue as so little dependent on the rest of the body. What we want to do first is to point out how this notion must affect our views of heredity. If the germinal tissue is simply harbored and nourished by the body, but otherwise quite independent of it, it follows as a matter of course that things the parent does, provided that they do not disturb the nourishment of the germinal tissue, can have very little effect upon it. For example, anything that the parent may achieve during his lifetime cannot be passed on by heredity to his offspring. This is entirely contrary to the idea of heredity which is held by nearly everybody. Just a little over one hundred years ago a Frenchman by the name of Lamarck published writings in which he argued very strongly in favor of the inheritance of “acquired characters,” meaning by this that the achievements of parents could be and were transmitted to the offspring. It is evident that this would be a very great advantage; the progress of the race would be secure if the children could inherit directly the achievements of their parents. Of course, it might work the other way around and children might inherit unfavorable acquisitions of their parents as well as favorable. Since the time of Lamarck there has been not only hot discussion of his ideas but also a vast amount of experimentation to find out whether acquired characters actually can be inherited. One of the most famous experiments is that of a man who cut off the tails of mice at birth and did this generation after generation in the hope of being able to produce a race of tailless mice; after a great many generations he abandoned the attempt with the conclusion that the artificial removal of tails from parents would not cause the offspring to be born in the tailless condition. More recently similar experiments have been carried out on kinds of animals that have much shorter lives and in which the generations are correspondingly more frequent. In some cases thousands of generations have been studied without any indication that changes brought about during the lifetime of the parents can be transmitted to the offspring.

One of the first questions that is sure to be asked as soon as this inheritance of acquired characters is denied, is how changes can then be brought about; the answer to that question is found in the experience of animal and plant breeders. One fact of nature which should be emphasized, is that individuals are never exactly alike except perhaps in the case of so-called identical twins which have a special heredity spoken of in Chapter V. Even brothers and sisters always have pretty marked differences. These differences among individuals are some of them probably more or less accidental results of the way in which the germ develops into the complete body; others are the result of differences in the germ cells themselves. The distinction between these is that where the germ cells are different, the difference will be transmitted by heredity to future generations; if it is a modification that comes on in connection with development and is not due to a difference in the germ cell, it will not show itself in future generations. What the animal and plant breeders have to do is to watch for changes in their stock, and when the kinds that they are looking for appear, they breed them in the hope that the desired features will prove to be hereditary, so that a race can be established showing them. A very striking example of this kind of breeding is in the development of hornless cattle. Of late years it has been the practice to dehorn cattle regularly and this dehorning has never caused the offspring to be born without horns, but on the other hand it has happened occasionally that calves have been born which were naturally hornless and since this absence of horns was due to a difference in the germ tissue, the character was found to be hereditary and it has been possible to establish breeds of hornless cattle. Thus by selecting cases where a change occurred naturally in the germ tissue, a result has been obtained which could not possibly be gotten by any amount of work directly upon the parents.