XXV. DIGESTIOX AXD 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 transported from one part of the plant body to another, . it is 



changed into a soluble form. In this state it can be passed from 



cell to cell by the process of osmosis. Much the same condition 



exists in animals. In order that food may be of use to man, it must 



„ h ,. ,/ „ be changed into a state that 



\ L^ ' •' will allow of its passage in a 



- ' soluble form through the walls 



^ " - of the alimentary canal, or 



f food tube. Digestion consists 



er.*^ ' '•'" ^^^ changing of foods from 



^ %. ■'' an insoluble to a soluble form, 



' \^ /M so that they may pass through 



L^Hm'"*"^**^ ^ ^ ■" ^'*^ walls of the alimentary canal 

 '■* -"^^ -^ ^ and become part of the blood. 



^ r r ^ ''' Prohlern XLV. Study of 



^V^ ^ ' ^^^ ^ fZzV'(",s!^7Z'p system of a frog 



A. H>^^*|^^ "'' orde?' better to understand 



*■ «- "■''^■i^ that of man. {.Lai/oratory 



"^ J -^P*' J Mamtal, Proh. XLV.) 



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 

 the gullet, stomach, small and 



I i 



Picture of the organs of digestion : a, in- 

 testine, leading out of the pylorus ; 6, liver ; 

 c, esophagus ; d, pancreas ; e, stomach ; 

 /, spleen ; g, i, j, k, m, n, parts of large 

 intestine ; h, I, small intestine. (From 

 Johonnot and Bouton.) 



pass from the mouth, posteriorly, 

 large intestine, and rectum. 



352 



