120 DIGESTION". 



membrane presents, also, a different structure, and has different 

 properties in different parts. In the mouth and oesophagus, it is 

 smooth, with a hard, whitish, and tessellated epithelium. This kind 

 of epithelium 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 projections on its free surface, which 

 are sometimes reticulated with each other. In the small intestine, 

 we find large transverse folds of mucous membrane, the valvulse 

 conniventes, the minute villosities which cover its surface, and the 

 peculiar glandular structures which it contains. Finally, in the 

 large intestine, the mucous membrane is again different. It is here 

 smooth and shining, free from villosities, and provided with a dif- 

 ferent glandular apparatus. 



Furthermore, the digestive secretions, also, vary in these different 

 regions. In its passage from above downward, the food meets 

 with no less than five different digestive fluids. First it meets with 

 the saliva in the cavity of the mouth ; second, with the gastric juice, 

 in the stomach ; third, with the bile ; fourth, with the pancreatic 

 fluid; and fifth, with the intestinal juice. It is the most important 

 characteristic of the process of digestion, as established by modern 

 researches, that different elements of the food are digested in different 

 parts of the alimentary canal by the agency of different digestive fluids. 

 By their action, the various ingredients of the alimentary mass are 

 successively reduced to a fluid condition, and are taken up 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 transforma- 

 tion, by which the ingredients of the food are altered in character 

 at the same time that they undergo the process of liquefaction. 

 The active agent in producing this change is in every instance an 

 organic matter, which enters as an ingredient into the digestive 

 fluid ; and which, by coming in contact with the food, exerts upon 

 it a catalytic action, and transforms its ingredients into other sub- 

 stances. It is these newly formed substances which are finally 

 absorbed by the vessels, and mingled with the general current of 

 the circulation. 



In our study of the process of digestion, the different digestive 

 fluids will be examined separately, and their action on the aliment- 

 ary substances in the different regions of the digestive apparatus 

 successively investigated. 



