308 



BIOLOGY 



of the food from the alimentary canal. Undigested foods are 

 not, as a rule, capable of osmosis. Digestion changes them 

 into a condition in which they are soluble and capable of os- 

 mosis. After complete digestion the foods in the alimentary 

 canal have been converted into a dialyzable liquid. More- 

 over, the structure of the intestine is such as to make osmosis 

 a natural process. This can be under- 

 stood from Figure 135, which illustrates 

 a diagrammatic cross section of the in- 

 testinal wall. In such a figure the food 

 occupies the space, in. The walls of the 

 intestine are thrown into little papillae 

 called villi, each of which is covered by a 

 membrane, m; on the other side of this 

 membrane, at bv, there are blood vessels 

 containing the blood, which is a liquid 

 of very different nature from the intes- 

 tinal contents. Thus it is seen that we 

 have a membrane separating two liquids 

 of different consistency, the blood on 

 the one side and the digestive food on 

 the other. Under these circumstances, 

 the force of osmosis will develop and the 

 material in the solution will begin at 

 once to pass through the membrane 

 from one side to the other. Thus the primary factor in the 

 absorption of food from the intestines is that of osmosis. 



The physical force of osmosis is not, however, the only factor 

 concerned in the absorption of food. If it were, there would 

 be an equivalent passage of liquid from the blood into the 

 intestine, as well as from the intestine into the blood. Such an 

 equivalent passage from the intestine does not seem to take 

 place, proving that the forces concerned in the absorption of food 

 are not confined to the process of osmosis. Moreover, a careful 

 study of the absorptive process shows that it is much more 



FIG. 135. DIAGRAM 

 SHOWING THE RE- 

 LATION OF PARTS IN 

 THE INTESTINE FOR 



THE ABSORPTION OF 

 FOOD 



bv, the blood vessels in the 

 intestinal wall; 



in, the intestinal cavity 

 occupied by the digested 

 food; 



m, the membrane of the 

 epithelial cell through which 

 the food dialyzes into the 

 blood vessels. 



