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24 poison if left in the blood, and do so if not perfectly removed.

When a permeable membrane interposes between a liquid and the air, an exchange takes place between the two. Although no pores can be seen in a bladder, water put in it will ooze through its walls and disappear by evaporation. Now the skin, in relation to the blood it contains, resembles a bladder full of hot fluid. Hence perspiration must always be going on through the substance of the skin itself; but the amount of this cannot be ascertained, owing to the fact that the skin is covered with the little glands, whose special duty is to excrete the perspiration.

All over the body tiny holes may be seen which are the openings of the sweat glands passing out through the scarf-skin. These glands are tiny tubes about 1/300th of an inch in diameter and a quarter of an inch long; they are coiled into a sort of knot, intermeshed with capillaries, and it is calculated that there are not less than two millions and a quarter or two millions and a half of them in the whole skin (see Fig. I). Their number varies in different parts of the body; they are fewest in the back and neck, where there are only about four hundred to the square inch, and more numerous on the palm of the hand and sole of the foot, where their openings may be seen through a magnifying-glass following the ridges on the skin, which are visible to the naked eye; a square inch of the skin of these parts contains from two to three thousand. The blood in the capillaries is only divided from