5 12 THE ABSORPTION OF FOOD. [CH. xxxiv. 



vein leave the spinal cord in the third to the eleventh thoracic 

 nerves inclusive (Bayliss and Starling). The nerves of the hepatic 

 artery are constrictors contained in the splanchnic, and dilatators 

 in both splanchnic and vagus. 



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 indigestible 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, consider- 

 ably 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 nitration. We must also take into account the fact that the 

 cells through which the absorbed substances pass are living, and 

 in virtue of their vital activity not only select materials for 

 absorption, but also change those substances while in" contact 

 with them. These cells are of two kinds (i) 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 



