One of the chief things in training for athletics is teaching our skin and heart together to get rid of the heat made by our muscles, as fast, or nearly as fast, as we make it, thus enabling us to keep on running, or working, without discomfort. As soon as we stop running, or working, the heart begins to slow down, the blood vessels in the skin contract and diminish in size, the flush fades, and we begin to cool off. We are not making either as much heat or as much waste as we were, and hence do not need to get rid of so much through our skins.
When we feel cold, just the opposite kinds of change occur in the skin. The blood vessels in the skin contract so as to keep as much of our warm blood as possible in the deeper parts of our body, and prevent its losing heat. As blood showing through the pavement-layer of the skin is what gives us our color, or complexion, our skin becomes pale and pasty-looking; and if all the blood is driven in from the surface, our lips and finger nails will become blue with cold. Here again, by changes in the skin, nature is simply trying to protect herself from the loss of too much heat.
If we exercise briskly, or eat a good warm meal, and thus make more heat inside of our body, then there is no longer any need to save its surface loss in this way; and the blood vessels in our skin fill up, the heart pumps harder, and the warm, rich color comes back to our faces and lips and finger nails.
So perfectly and wonderfully does this skin mesh of ours work, by increasing or preventing the loss of heat, that it is almost impossible to put a healthy man under conditions that will raise or lower his temperature more than about a degree, that is to say, about one per cent above, or below, its healthful level. Men studying this power of the skin have shut themselves into chambers, or little rooms, built like ovens, with a fire in the wall or under the floor, and found that if they had plenty of water to drink and perspired freely, they could stand a temperature of over 150° F. without great discomfort and without raising the temperature of their own bodies more than about one degree. If, however, the air in the chamber was moistened with the vapor of water, or steam, so that the perspiration could no longer evaporate freely from the surface of their bodies, then they could not stand a temperature much above 108° or 110° without discomfort.
Other men, who were trained athletes, have been put to work in a closed chamber, at very vigorous muscular exercise, so as to make them perspire freely. But while a thermometer placed in that chamber showed that the men were giving off enormous amounts of heat to the air around them, another thermometer placed under their tongues showed that they were raising the temperature of their own bodies only about half a degree. One man, however, happened to try this test one morning when he was not feeling very well, and didn't perspire properly, and the thermometer under his tongue went up nearly four degrees.
THE NERVES IN THE SKIN
How We Tell Things from Touch, and Feel Heat and Cold and Pain. Last of all, the skin is the principal organ of the sense of touch, and also of the "temperature sense"—the sense of heat and cold—and of the sense that feels pain. All these feelings are attended to by little bulbs lying in the deeper part of the skin and forming the tips of tiny nerve twigs,[20] which run inward to join larger nerve branches and finally reach the spinal cord. There are millions of these little bulbs scattered all over the surface of the skin, but they are very much thicker and more numerous in some parts than in others; and that is why, as you have often noticed, certain parts of the skin are more sensitive than others. They are thickest, for instance, on the tips of our fingers and on our lips, and fewest over the back of the neck and shoulders, and across the lower part of the hips.[21]
For a long time, it was supposed that all these little nerve-bulbs in the skin did the same kind of work, because they looked, under the microscope, exactly alike; but it was found that they divide the work up among them, so that some of them give their entire attention to heat, and others to cold, others to touch, and others again to pain. So carefully has the work been mapped out among them that they report to different centres in the brain and spinal cord, so that we now understand why, in diseases which happen to attack one or other of these centres, we may lose our sense of heat and cold, as in that terrible disease, leprosy; or our sense of touch, as in paralysis; or we may even, in some very rare cases, lose our sense of pain, and yet have all our other senses perfect.