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 ridge in the skin above each row of papillæ (Fig. 24). In the papillæ are small loops of blood vessels and sometimes a nerve fiber (colored Fig. 1).

The epidermis is composed of a mass of cells held together by a cement resembling the white of an egg. The cells near the surface are hard and flattened; those deeper down near the dermis are round and soft (see Fig. 21). These cells are living cells. They are kept alive by the nourishment in the watery portion of the blood which soaks through from the blood tubes in the neighboring papillæ. Hence these cells are growing cells; they subdivide when they reach a certain size, and replace those wearing away at the surface, thus constantly repairing the epidermis. The dry outer cells wear away rapidly. They have no nuclei and are dead cells. The new cells forming beneath push them so far away from the dermis that nourishment no longer reaches them, and they die.

Pigment.—The cells in the lower layers of the epidermis contain grains of coloring matter, or pigment. All other cells of the epidermis are transparent; the pigment has the function of absorbing and arresting light. Albinos or animals entirely without pigment have pallid skins and pink eyes (Exp. 1).