378 



HOW FOOD IS PREPARED FOR BODY USE 



the intestine. Underneath these cells lies a network of very fine 

 blood vessels and in the center of the villi are spaces which, because 



thoracic duct 



Superior vena cava 



of their white ap- 

 pearance after the 

 absorption of fats, 

 have been called 

 lacteals. 



Absorption of 

 foods. While diffu- 

 sion and osmosis are 

 important factors in 

 the passage of food 

 and water through 

 the walls of the in- 

 testine, most physi- 

 ologists agree that 

 the living matter in 

 the cells lining the 

 intestine exerts 

 energy that affects 

 the absorption of the 

 substances that pass 

 into the blood and 

 lacteals. This is 

 proven by the fact 

 that if these cells are 

 injured or poisoned, 

 then absorption fol- 

 lows the laws of 

 osmosis and diffusion. Ordinarily the cells lining the intestine are 

 like tiny chemical laboratories. Since the object of digestion is to 

 furnish the cells with building material as well as energy foods, it 

 is evident that amino acids, after having been absorbed into the 

 blood, can get into the cells by a similar process. Fats, for example, 

 in the form of fatty acids and glycerol, enter the epithelial cells of 

 the villus and during the process of passing through them are 

 changed back into fat particles. 



lrrnpHcxtics- 



Explain from the diagram and text how the various nutrients 

 reach the blood. 



