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CHAP, II.] go on in the lesser and greater circulation are, in a manner, the reverse of one another.

The skin acts as an excretory organ, much in the same way as the lungs and kidneys do; in each of these organs water, carbonic acid, and dissolved matters pass through the animal membrane of the capillaries, but the three differ in the absolute and relative amounts of their excretions. In all three water is the chief constituent, while most solid matter is given off by the kidneys, and most gaseous matter by the lungs. The skin has something of the nature of both lungs and kidneys; it takes in oxygen, and breathes out carbonic acid and water, like the lungs do, and it excretes organic and saline matter dissolved in water, as do the kidneys, with which it is so closely related, that when its action is interfered with, the kidneys do some of its work, and vice versâ. When the excretion of the skin is increased, as in hot weather, that of the kidneys is diminished; and when, as in cold weather, less work is done by the skin, the kidneys are brought into more active service. The skin is composed of two layers—the outer or scarf-skin (otherwise called the cuticle or epidermis), and the inner so-called true skin, cutis or dermis. The outer layer varies much in thickness at different parts of the body, being thickest where there is most pressure, as on the palms of the hands and the soles of the feet; it is dead material, containing neither nerve-fibres nor bloodvessels, and is in the form of layers of fine, flat cells or scales laid one over another, like tiles on a roof. These scales