DIGESTION. 135 



the small intestine (e), different parts of which, owing to the vary- 

 ing structure of their mucous membrane, have received the different 

 names of duodenum, jejunum, and ileum. In the duodenum are situated 

 the orifices of the biliary and pancreatic ducts (f, g). Finally comes 

 the large intestine (h, i,j, &), separated from the smaller by the ileo-caecal 

 valve, and terminating, at its lower extremity, by the anus, at which is 

 situated a double sphincter, for the purpose of guarding its orifice. 

 Everywhere the alimentary canal is composed of a mucous membrane 

 and a muscular coat, with a layer of submucous connective tissue between 

 the two. The muscular coat is composed of a double layer of longitudinal 

 and transverse fibres, by the alternate contraction and relaxation of which 

 the food is carried through the canal from above downward, and the 

 arrangement of which varies indifferent portions of the alimentary tract. 

 The mucous membrane presents, also, a different structure, and has dif- 

 ferent properties in different parts. That of the mouth and oesophagus 

 is smooth, with a hard, white, tessellated epithelium, which, however, 

 terminates abruptly at the cardiac orifice of the stomach. The mucous 

 membrane of the gastric cavity is soft and glandular, covered with a 

 transparent, columnar epithelium, and thrown into minute folds or pro- 

 jections on its free surface, which are sometimes reticulated with each 

 other. In the small intestine it presents larger transverse folds known 

 as the " valvulse conniventes," is covered upon its free surface with 

 thickly set villosities of various forms, and contains throughout an 

 abundance of tubular follicles. Finally, in the large intestine the mu- 

 cous membrane is smooth and shining, free from villosities, and pro- 

 vided with a glandular apparatus different in structure and function 

 from that of the preceding parts. 



The digestive secretions, also, vary in these different regions. In its 

 passage from above downward, the food meets with at least five dif- 

 ferent secreted fluids, namely, the saliva, in the cavity of the mouth: 

 the gastric juice in the stomach; and the intestinal juice, with the pan- 

 creatic juice and the bile, discharged into the cavity of the small intes- 

 tine. These fluids are themselves, in some instances, of complex nature, 

 resulting from the mingled secretions of several different associated 

 glands, or of the various parts of a single mucous membrane. To a 

 certain extent, the special action of each digestive fluid upon the food 

 has been investigated ; and it is found that certain of the secretions 

 have a distinct and peculiar influence upon special ingredients of the 

 food. As the result of the successive action of the digestive fluids, 

 modified, perhaps, by the effect of their combined operation, the sub- 

 stances composing the alimentary mass are gradually reduced to a fluid 

 condition, in which they are fit for absorption by the vessels of the 

 intestinal mucous membrane. 



The action which is exerted upon the food by the digestive fluids is 

 not that of a simple chemical solution. It is a transformation, by 

 which the ingredients of the food are altered in character at the same 



