XXVIII. DIGESTION AND ABSORPTION 



Purpose of Digestion. — We have learned that starch and proteid 

 food of plants are formed in the leaves. A plant, however, is 

 unable to make use of the food in this condition. Before it can 



be used it is changed into a 

 soluble form, such as grape 

 sugar. In this state it can 

 be passed from cell to cell 

 by the process of osmosis, 

 and can be used to build 

 new cells or to release energy. 

 The same condition exists in 

 animals. In order that food 

 may be of use to man, it 

 must be changed into a state 

 that will allow of its passage 

 in a soluble form through 

 the walls of the alimentary 

 canal or food tube. Diges- 

 tion consists in the changing 

 of foods from an insoluble to 

 a soluble form, so that they may 

 pass through the walls of the 

 alimentary canal and become 

 part of the blood. 



Alimentary Canal. — In all 

 vertebrate animals, including 

 man, food is normally taken 

 in the mouth and passed through a food tube during the process 

 of digestion. This tube is composed of different portions, named, 

 respectively, as we pass from the mouth, posteriorly, the gullet, 

 stomach, small and large intestine, and rectum. 



330 



m- 



Picture of the organs of digestion; a, intestine, 

 leading out of the pylorus; 6, liver; c, esoph- 

 agus; d, pancreas; e, stomach; /, spleen; 

 g, i, j, k, m, n, parts of large intestine; 

 h, I, small intestine. From Johonnot and 

 Bouton. 



