[pg 139]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—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[pg 140] 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—Diagram of the peritoneum. 1. Transverse colon. 2. Duodenum. 3. Small intestine. 4. Pancreas.
The Peritoneum.—The peritoneum is to the abdominal cavity what the pleura is to the thoracic cavity. It forms the outer covering for the alimentary canal and other abdominal organs and supplies the inner lining of the cavity itself. It is also the means of holding these organs in place, some of them being suspended by it from the abdominal walls (Fig. 64). By the secretion of a small amount of liquid, it prevents friction of the parts upon one another.