III. ABSORPTION OF FOOD 



1. State in which Food leaves the Alimentary Canal. The 



carbohydrates generally leave the alimentary canal as 

 monosaccharids ; but some resist the action of digestion 

 more than others. Lactose seems to be broken down in the 

 intestine only when the special lactase is present in the succus 

 entericus, but in all cases it must be broken down before it 

 reaches the liver. Cane sugar when taken in large excess 

 may also be absorbed and it is then excreted by the kidneys. 



The proteids are absorbed as peptones, possibly as proteoses, 

 and as the diamido acids and other crystalline compounds 

 formed by the action of erepsin (p. 350). Native proteids 

 may be absorbed unchanged from the lower bowel, since 

 it has been found that when egg white is injected into an 

 isolated part of the rectum it disappears to a very consider- 

 able extent. 



The fats are chiefly absorbed as soaps and as fatty acids, 

 and it is very doubtful if they leave the gut as fats. 



2. Mode of Absorption of Food. Absorption does not 

 occur uniformly throughout the alimentary canal. Thus, 

 while sugar and peptones are absorbed from the stomach, 

 water is absorbed only to a small extent. 



That absorption is not due merely to a process of ordinary 

 diffusion or osmosis is clearly indicated by many facts. 



1. Heidenhain has shown that absorption of water from 

 the intestine takes place much more rapidly than diffusion 

 through a dead membrane. 



2. The relative rate of absorption of different substances 

 does not follow the laws of diffusion. Griibler's peptone passes 

 more easily through the intestine than glucose, but glucose 

 passes more readily through parchment paper, while sodium 



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