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Arthur C. Custance, Ph.D. |
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In the Sweat of Thy Brow |
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1. Animal and Human Sweating (cont.)In other words, while it may appear that sweating is a kind of universal reaction, the different kinds of sweating are actually quite differently controlled by the central nervous system and may work not only in entire independence but even in antagonism. Let me give two illustrations of this. A person brought into an experimental laboratory who is not familiar with procedures, and who is to be subjected to a rising temperature in the room, may very quickly respond to the situation with an outburst of emotional sweating resulting from anxiety. Since the body is still cool enough, there will be no thermal sweating. As the temperature rises, |
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however, in due time thermal sweating breaks out, but in the meantime the subject has become more at ease and mental and emotional sweating subside. However, if the room temperature is raised high enough so that the subject is distressed, emotional sweating breaks out once again. Throughout this whole process the course of these two different kinds of sweating in specific areas of the body can be measured and recorded continuously and are found to be virtually independent of one another. A second example is seen in the response of the sweat glands to a loss of consciousness as a subject goes to sleep or is anaesthetized. In normal sleep all mental sweating in the palmar regions ceases but, for reasons that are not well understood, there may be a considerable increase in the thermal sweating. The two curves cross over as they follow their entirely different course, the one declining as the other rises. In this brief survey of the subject, we have spoken of mental and emotional sweating as occurring together. This may not be so at all, each taking place independently. Thus mental arithmetic, as already mentioned, elicits mental sweating even in children without any necessary emotional component. Fright may induce emotional sweating in the axillae without any mental stimulation. Extreme fright may, in fact, block mental activity altogether. Just to keep the picture clear, we may summarize by saying that there are here three kinds of sweating: mental, emotional, and thermoregulatory. Sometimes these occur together, sometimes they occur independently. In some areas the glands appear to have the capability of sweating in response to two different kinds of stimuli: in the palmar area, mental and emotional; and in the axillae, emotional and thermal. There is one unique area of the skin surface in which all three kinds of sweating may occur singly or together. This is in the forehead region. We return to this in the last chapter. Now, it should be emphasized that these different kinds of response do not result from the fact that the brain merely sends out an appropriate message and the gland reacts in an appropriate way, all messages travelling along the same lines of communication. In reality there are different lines of communication (nerve pathways) for the different kinds of sweating response. So any single area which can respond in two different ways probably has two different lines of communication, and these lines of communication do not originate in the same area of the brain nor do they leave the spinal cord at the same level. That one area the forehead region where three kinds of response are possible may perhaps be supplied with three systems of communication with the central nervous system. These anatomical features are not fully understood yet, and the authorities have by no means reached complete agreement. However, what has been said above regarding the innervation of the various types of sweat glands is sufficiently true to serve the purpose of making it very clear that these differences are not merely functional but anatomical also. With respect to animals, the situation is complicated because sweat gland function appears to serve the same object but in fact almost certainly does not. Thermoregulatory sweating in those animals which can be made to sweat in response to heat seems to be an unnatural phenomenon, and it is possible that the outbreak of sweat is not so much directed toward the prevention of a rise in body temperature (though it incidentally serves this purpose) as it is an indication of distress. The reason for believing this is that the sweat glands of horses, for example, are apocrine in nature, i.e., similar to the glands in the human axillae and are anatomically part of the sympathetic nervous system which, it may be remembered, does not provide for body cooling. Those who love beautiful animals (and I most certainly do) will object that a horse can sweat profusely and can then catch cold as a consequence. True, but there are two important observations to be made about this. First, sweating in horses is the result of exercise, not of a rise in body temperature. What triggers the sweat gland activity is a rise in adrenalin in the blood, not a rise in temperature in the body as a whole. The gland activity is not strictly thermogenic and only serves to cool the body by accident. Secondly, it is an inefficient system in any case, because the horse does not have any concurrent peripheral vasodilatation that would increase the transfer of heat to the skin surface, and its comparatively long hair makes even evaporative cooling inefficient. In nature, it is doubtful whether a horse ever sweats through the skin in order to maintain a normal body temperature. Sweating may possibly serve some other function, as a bactericide or fungicide on the skin, for example. It is analogous to the discomfort bordering upon pain which a high temperature may cause ahuman being, resulting in emotional sweating under the arms. Only a few animals actually sweat at all under normal circumstances in response to a rise in temperature. As one authority has pointed out, on a very hot, humid summer day it is the visitors at the zoo, and not the animals, who sweat. The sweating of horses is an exceptional circumstance, since it results from their being driven against their natural inclination to a level of exercise which they would otherwise most certainly avoid. It is a curious thing how frequently the theory of evolution has led to disappointing results when used as a guide in research. James Hardy, one of the most prominent physiologists to study the thermoregulation of the body, points out that at one stage of his research cebus monkeys were chosen as substitutes for humans in the belief that their thermoregulatory mechanisms would be much nearer those of humans than would other animals, such as the cat or dog. Research proved this to be quite wrong, the monkey being particularly poorly equipped to prevent a temperature rise. He concluded that "the cebus monkey is not a good experimental animal for bridging the gap between the data available on man and that available on animals as regards temperature regulation." What does appear from Hardy's work and from other sources is that the few animals that have some ability to prevent a temperature rise by evaporative cooling can be arranged in an ascending order of efficiency as follows. The monkey has virtually no ability, the cat has slight ability, the dog has greater ability still, the horse even more, and man almost complete. Somewhere between the dog and man must be placed cattle. The interesting thing here is that the ability to regulate body temperature by evaporative processes appears to increase according to the level of domestication enjoyed by the species, the monkey being least so, the cat next, and so on. Some dog lovers would claim greater domestication for the dog than the horse, but in those conditions under which sweating is likely to be necessary - i.e., a high work rate - the horse is undoubtedly better domesticated than the dog. Few would question that a harnessed horse is normally more completely tractable than a harnessed dog. I would conclude from all this that in nature no animal even begins to approach the level of physical efficiency of man's evaporative cooling system, least of all these particular monkeys. As we shall see, the uniqueness of man in this respect results partly from the fact that he is a fallen creature with a bodily efficiency that is very low, probably not more than 20 percent. We shall examine later why he sweats on this account. Before we leave this general consideration of the subject, it may be of interest to note that animals have numerous ways of keeping themselves cool. Some small rodents are supplied with ample spittle and wash themselves thoroughly if they are feeling too warm, the evaporation of the spittle providing them with sufficient cooling. Some other small animals are supplied with a specially structured tail which acts as a heat exchanger when positioned in a certain way and allows them to reduce body heat. The sweat glands of dogs and similar creatures are in the mouth, and the rapid exchange of air in panting provides them with sufficient evaporative cooling. Prehistoric animals, like the dimetrodon, were equipped with a sail-like structure on the back which served probably as a heat exchanger, acting in two ways: to remove body heat in hot weather and to gain solar heat in the cold. Some animals make the adjustment by reducing basal metabolism, while others immerse themselves in water or burrow into the cool ground. Birds lift their wings slightly. Whatever the means, the fact remains that there are limitations which generally restrict the species to a certain temperature zone. Man is virtually free of such restrictions, partly by reason of his ability to produce an artificial environment by clothing and shelter, but also because of his ability to resist a temperature rise by copious sweating and the consequent evaporative cooling. Not infrequently this copiousness may seem to be wasteful. But a considerable body of evidence exists to show that the few elements which do exist in the sweat are of importance to man, since his body is thereby washed with a slightly acid solution which serves to protect him against bacteria and fungi. Unevaporated sweat, which seems to be an overcompensation and a pure waste of body fluid, turns out to have a value of its own. In the one area of the body where the acid might be dangerous to himself, namely, in the forehead region, the sweat is prevented from running down into his eyes by the eyebrows. As Scripture says, truly we are "wonderfully and fearfully made".
Next Chapter - The Sweating of Fallen Man
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More Writings of Arthur Custance |
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