/> a wide surface throughout the body, that they are actually capable of

holding, at one time, nearly one-tenth of all the blood in the body.



This water-jacket coat of tiny blood vessels all over our body has

some very important uses: It allows the heart to pump large amounts of

blood out to the surface to be purified by the sweat glands, and to

breathe out a little of its carbon dioxid and other gas-poisons.



The Skin as a Heat Regulator. Heat, as well as waste, is given off by

the blood when it is poured out to the surface; so another most

important use of the skin is as a heat regulator. As we have already

seen, every movement which we make with our muscles, whether of arms and

limbs, heart, or food tube, causes heat to be given off. We very well

know, when we work hard at anything, we are likely to get warmed up.

Although a certain amount of this heat is necessary to our bodily

health, too much of it is very dangerous.



Just as it is best for the temperature, or heat, of a room to be at

about a certain level, somewhere from 60 deg. to 70 deg. F., so it is

best for the interior of our bodies to be kept at about a certain heat.

This, as we can show by putting a little glass thermometer under the

tongue, or in the armpit, and holding it there for a few minutes, is a

little over 98 deg. F. (98.4 deg. to be exact); and this we call body

heat, or blood heat, or normal temperature. Our body cells are, in

one way, a very delicate and sensitive sort of hot-house plants, though

tough enough in other respects. Whenever our body heat goes down more

than five or six degrees, or up more than two or three degrees, then

trouble at once begins. If our temperature goes down, as from cold or

starvation, we begin to be drowsy and weak, and finally die. If, on the

other hand, our temperature climbs up two, three, or four degrees, then

we begin to be dizzy and suffer from headache and say we have a fever.



A fever, or rise of temperature, that can be noted with a thermometer,

is usually due to disease germs of some sort in the body; and most of

the discomfort that we suffer is really due more to the poisons (toxins)

of the germs than to the mere increase of heat, though this alone will

finally work serious damage. However, as we well know from repeated

experience, we need only to run or work hard in the sun for a

comparatively short time to make ourselves quite hot enough to be very

uncomfortable; and if we had no way to relieve ourselves by getting rid

of some of this heat, we should either have to stop work at once, or

become seriously ill. This relief, however, is just what nature has

provided for in this thick coat of blood vessels in our skin; it enables

us to throw great quantities of blood out to the surface where it can

get rid of, or, as the scientists say, radiate, its heat. This cooling

process is hastened by the evaporation of the perspiration poured out at

the same time, as we have seen.



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 deg. 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 deg. or 110 deg. 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.