CHAPTER XXXIV 



THE ABSORPTION OF FOOD 



FOOD is digested in order that it may be absorbed. It is absorbed in 

 order that it may be assimilated, that is, become an integral part of 

 the living material of the body. 



The digested food thus diminishes in quantity as it passes along 

 the alimentary canal, and the faeces contain the undigested or indi- 

 gestible residue. 



In the mouth and oesophagus the thickness of the epithelium and 

 the quick passage of the food through these parts reduce absorption 

 to a minimum. Absorption takes place more rapidly in the stomach : 

 the small intestine with its folds and villi to increase its surface is, 

 however, the great place for absorption ; and although the villi are 

 absent from the large intestine, absorption occurs there also, but to 

 a less extent. 



Foods such as water and soluble salts like sodium chloride are 

 absorbed unchanged. The organic foods are, however, considerably 

 changed, colloid materials like starch and proteid being converted 

 respectively into the diffusible materials sugar and peptone. 



There are two channels of absorption, the blood-vessels (portal 

 capillaries) and the lymphatic vessels or lacteals. 



Absorption, however, is no mere physical process of osmosis and 

 filtration. We must also take into account the fact that the cells 

 through which the absorbed substances pass are living, and in virtue 

 of their inherent activity not only select materials for absorption, but 

 also change those substances while in contact with them. These cells 

 are of two kinds (1) the columnar epithelium that covers the surface ; 

 and (2) the lymph cells in the lymphoid tissue beneath. It is now 

 generally accepted that of the two the former, the columnar epithelium, 

 is the more important. 



Absorption of Carbohydrates. Though the sugar formed from 

 starch by ptyalin and amylopsin is maltose, that found in the blood 

 is glucose. Under normal circumstances little, if any, is absorbed by 

 the lacteals. The glucose is formed from the maltose by the succus 

 entericus, aided by the action of the epithelial cells through which it 



