ORGANS AND PROCESSES OF DIGESTION 



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 : 



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 struc- 

 ture it resembles the skin, being 

 continuous with the skin where cavi- 

 ties open to the surface. It is made 

 up of two layers a thick under- 

 layer which contains blood vessels, 

 nerves, and glands, and a thin sur- 

 face layer, called the epithelium. 

 The epithelium, like the cuticle, is 

 without blood vessels, nerves, or 

 glands. 



2. A middle coat, which is mus- 

 cular 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 



10 



FIG. 63. Diagram of 

 the digestive system. 

 I. Mouth. 2. Soft palate. 

 3. Pharynx. 4. Parotid gland. 

 5. Sublingual gland. 6. Sub- 

 maxillary gland. 7. Esoph- 

 agus. 8. Stomach. 9. Pan- 

 creas. 10. Vermiform ap- 

 pendix, n. Caecum. 12. 

 Ascending colon. 13. Trans- 

 verse colon. 14. Descend- 

 ing colon. 15. Sigmoid flex- 

 ure. 1 6. Rectum. 17. Ileo- 

 csecal valve. 18. Duct from 

 liver and pancreas. 1 9. Liver. 



Diagram does not show 

 comparative length of the 

 small intestine. 



