*The Alimentary Canal. *—The alimentary canal has a length of about thirty feet and, while it begins at the mouth, all but about eighteen inches of it is found in the abdominal cavity. On account of its length it lies for the most part in coils, the two largest ones being known as the small intestine and the large intestine. Connected with the alimentary canal are the glands that supply the liquids for acting on the food. The divisions of the canal and most of the glands that empty liquids into it are shown in Fig. 63 and named in the table below: [Table] *Coats of the Alimentary Canal.*—The walls of the alimentary canal, except at the mouth, are distinct from the surrounding tissues and consist in most places of at least three layers, or coats, as follows: [Fig. 63] Fig. 63—*Diagram of the digestive system.* 1. Mouth. 2. Soft palate. 3. Pharynx. 4. Parotid gland. 5. Sublingual gland. 6. Submaxillary gland. 7. Esophagus. 8. Stomach. 9. Pancreas. 10. Vermiform appendix. 11. Cæcum. 12. Ascending colon. 13. Transverse colon. 14. Descending colon. 15. Sigmoid flexure. 16. Rectum. 17. Ileo-cæcal valve. 18. Duct from liver and pancreas. 19. Liver. Diagram does not show comparative length of the small intestine. 1. An _inner coat_, or lining, known as the mucous membrane. This membrane is not confined to the alimentary canal, but lines, as we have seen, the different air passages. It covers, in fact, all those internal surfaces of the body that connect with the external surface. It derives its name from the substance which it secretes, called _mucus_. In structure it resembles the skin, being continuous with the skin where cavities open to the surface. It is made up of two layers—a thick underlayer which contains blood vessels, nerves, and glands, and a thin surface layer, called the _epithelium._ The epithelium, like the cuticle, is without blood vessels, nerves, or glands. 2. A _middle coat_, which is muscular and which forms a continuous layer throughout the canal, except at the mouth. (Here its place is taken by the strong muscles of mastication which are separate and distinct from each other.) As a rule the muscles of this coat are involuntary. They surround the canal as thin sheets and at most places form two distinct layers. In the inner layer the fibers encircle the canal, but in the outer layer they run longitudinally, or lengthwise, along the canal.(57) 3. An _outer_ or _serous coat_, which is limited to those portions of the canal that occupy the abdominal cavity. This coat is not found above the diaphragm. It is a part of the lining membrane of the cavity of the abdomen, called [Fig. 64] Fig. 64—*Diagram of the peritoneum.* 1. Transverse colon. 2. Duodenum. 3. Small intestine. 4. Pancreas.