714. Cutaneous membrane, or the skin, which forms the external covering of the body, is an organ in which manifold secretions are constantly elaborated; but the skin is only a modification of the membrane which lines the interior of the body, the mucous. Mucous membrane forms the basis of the secreting apparatus placed in the mouth, fauces, esophagus, stomach, and intestines in their whole extent; of the secreting apparatus auxiliary to that of the alimentary canal, namely, the pancreas and the liver; probably also of the mesenteric, or lacteal glands, together with the vast system of lymphatic glands, and certainly of the glands of the larynx, trachea, bronchi and air vesicles of the lungs. Hence, while membrane forms the basis of the secreting apparatus in general, mucous membrane is far more extensively employed in its construction than any other form of membrane. 715. 1. In the construction of the secreting apparatus, membrane disposed in the simplest form, constitutes merely a uniform, smooth, extended surface. Serous membrane is always disposed in this simple mode. The costal pleura which lines the internal surface of the walls of the chest (fig. LX. a); the pulmonary pleura which is continued from the walls of the chest over the lungs (fig. LX. 5); the peritoneum which lines the internal surface of the cavity of the abdomen, and which is reflected over the viscera contained in it (fig. LX. c, and 6, 7, 8, &c.); the synovial membrane which covers all the articular surfaces; the arachnoid membrane which envelopes the brain, form simple continuous, serous, secreting surfaces. On the contrary, mucous membrane is never disposed in this perfectly simple mode; even when it forms a continuous surface, as in the lining, which it affords to the alimentary canals, it is more or less plaited into folds or rugæ (fig. CLXVII. 1). [Illustration: Fig. CLXXXI. A portion of the mucous surface of the intestines, showing some of the mucous glands which present the appearance of fovæ or cryptæ.] 716. 2. The second disposition of membrane in the construction of the secreting apparatus, is the depression of it into a minute pit or fova, called a crypt (CLXXXI.), which is sometimes inclosed on all sides, forming a cell or vesicle (fig. CXXXVIII.). [Illustration: Fig. CLXXXII. Portion of the skin and cellular tissue, showing the sebaceous follicles, as seen under the microscope very highly magnified. 1. The external surface of the follicles with the blood-vessels ramifying upon it. 2. Follicles laid open, showing the interior cavity into which the secreted fluid is poured.] 717. 3. Next, the vesicle, instead of being rounded, is elongated into a peduncle or neck, not unlike the neck of a bottle (fig. CLXXXII. 1). This pedunculated vesicle is called a follicle. 718. 4. Then, the follicle is somewhat elongated, without neck and without terminal expansion (fig. CLXXXVI. 1); and this is called a cæcum or pouch.