ABSOKPTION OF FOOD. 275 



Absorption of Digested Food. Water and inorganic com- 

 pounds, e.g., common salt, are absorbed without change. 

 Little or no absorption takes place in the mouth or gullet. It 

 commences in the stomach, but is mainly accomplished in the 

 intestines. The absorption is not merely a physical process of 

 diffusion ; the living cells through which it occurs exercise a 

 selective action and in many cases produce important chemical 

 changes in the substance being absorbed. 



Absorption of carbohydrates. These are probably entirely 

 absorbed as glucose, the necessary changes being produced in 

 the food by the enzymes in the saliva, pancreatic juice, and in- 

 testine as already described. A portion at once enters the 

 blood stream and is conveyed to the tissues, while another 

 portion is probably stored as a reserve in the liver in the form 

 of glycogen. 



Absorption of fats. A small portion of the fat in the food is 

 saponified ; but, as already stated, the greater portion is merely 

 emulsified. The lacteals of the intestine are the absorbents 

 for fat, and exactly how the minute globules pass through the 

 walls of the intestine is not understood. When fatty acids 

 are fed to an animal the contents of the lacteals contain fat, so 

 that glycerine must have come from some other portion of the 

 food and fat must be synthesised. 



Absorption of proteids. Soluble proteids are often absorbed 

 unchanged ; in general, however, the proteids are converted 

 into peptones and albumoses by the action of either pepsin 

 or trypsin. The extraordinary fact is that in the blood no pep- 

 tones can be found ; indeed, if they be injected into the blood 

 stream, poisonous effects are at once produced. The epithelial 

 cells must, during absorption, change the peptones and 

 albumoses back again into the original proteids. 



The length of the alimentary canal varies greatly in different 

 animals, being comparatively short in carnivora and very long 

 in herbivora. The usual length of the intestines of the various 

 farm animals and the percentage ratio of the capacity of the 

 stomach to the total capacity of stomach and intestines are as 

 follows : 



T2 



