In the skin, these little gland-pockets are of two kinds, the _sweat glands_ and the _hair glands_.
The sweat glands are tiny tubes which go twisting down through the different pavement layers, through the basement layer, and right into the coat of fat, which lies just under the skin.

The tube of the sweat gland soaks, or picks, out of the blood some of the waste-stuff--just as the kidney tube does in the kidney,--together with a good deal of water and a small amount of delicate oil, and pours them out on the surface of the body in the form of the "sweat," or _perspiration_.
As you will remember, when the muscles work hard and pour more waste into the blood, then the heart pumps larger amounts of blood out into the skin; and this causes it to redden.

The sweat glands work harder to purify this extra blood, and they pour out the waste and oil and water on the surface.

As soon as this water gets upon our hot skin, it begins to evaporate and cool us off, as well as to carry off some of the waste in the form of gas.

The trace of oil in the perspiration helps to lubricate the skin and keep it soft; but when too much of it is poured out we have that greasy feeling, which we have all felt after perspiring freely.
From all this cooling and breathing and blood-purifying work going on upon the surface of our skin, you can easily see why it is so important that all our clothing should be loose and porous and that next the skin easily washed; else it will very soon become clogged up and greasy, and shut off the breathing and blood-purifying work of the skin and make it dirty and unhealthy.