This membrane is to the cavities which it lines, what the skin is to the surface of the body. It is an internal skin, which is a continuation of the external Like the skin, it is an organ of absorption and secretion. It is composed of a corium or true skin and a sort of epidermis, named the epithelium, variable in its texture and in its elements according as it is to offer more or less resistance. A peculiar fluid—mucus—is secreted by this membrane and preserves its softness. The mucous membrane is covered, throughout the digestive canal, even to the end of the small intestine, with great numbers of papillæ or villi, and especially on the tongue. In the stomach it has numerous folds, which are effaced when the organ is distended; and throughout the smaller intestine it forms the valvulae conniventes, wrinkles or folds designed to increase the extent of absorbing surface.