The whole exterior surface of the body is covered by the skin, an excreting and absorbing organ, which serves as a protection to the parts beneath and is also the organ of touch. It has two layers, a superficial and a deep. The superficial layer, the epidermis or cuticle, is composed wholly of epithelial cells, of which the deepest layer is columnar and moulded upon the papillary layer of the derma, while the intermediate layers are more rounded and the surface ones flat. The deepest layer also contains the skin pigment, which causes the variation in shade between the Indian, the negro, and the white man. Below the epidermis, which is chiefly protective, is the tough, elastic, and flexible tissue of the derma or true skin, in which are vested most of the activities of the skin. Its surface is covered with papillæ, which are more numerous in the more sensitive parts. Each papilla contains one or more capillary loops and one or more nerve fibers, while some terminate in an oval body known as a tactile corpuscle. Beneath the papillæ is the reticular layer, composed of interlacing bands of fibrous tissue and containing blood-vessels, lymphatics, and nerves, as well as unstriped muscle fibers where hair is present. [Illustration: FIG. 11.--Vertical section of skin.] At the apertures of the body the skin stops and is replaced by _mucous membrane_, an integument of greater delicacy but which consists fundamentally of the same two layers, a superficial, bloodless epithelium and a deep fibrous derma. It is continuous with the skin, but is much redder and more sensitive and bleeds more easily. The passages and cavities that it lines, unlike those lined by serous membranes, communicate with the exterior of the body and are for that reason protected against contact with foreign substances by mucus, which is thicker and more sticky than the lymph that moistens the endothelium found on serous surfaces. Mucous membrane is found in the alimentary canal, the respiratory tract, and the genito-urinary tract. In cavities, like the stomach and intestines, which are subject to variations in capacity, it is thrown into folds or rugæ. The mucus is secreted by small glands in the membrane. =Appendages of the Skin.=--The skin has various appendages. On the dorsal surface of the last phalanges of the fingers and toes are flattened and horny modifications of epithelium, the _nails_. They have a root embedded in a groove of skin by which they grow in length and a vascular matrix of derma beneath them which gives growth in thickness. To their growth in length there seems to be no limit.