CHAPTEK XXXVI 



THE ABSORPTION OF 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 very slightly in the stomach. 

 The most recent observations show that water is not absorbed 

 in the stomach, but alcohol is absorbed to some extent. Salts 

 also do not seem to be absorbed unless present in great concentra- 

 tions, such as do not occur in normal diets ; sugar is absorbed with 

 difficulty. The small intestine, with its folds and villi to increase its 

 surface, is the great place for absorption.* Absorption begins in the 

 duodenum, and the products of digestion have largely disappeared by 

 the time the intestinal contents reach the ileo-caecal valve at the 

 commencement of the large intestine ; in the large intestine, absorp- 

 tion (mainly of water) occurs 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 such as starch and protein being converted 

 respectively into the diffusible materials sugar and ammo-acids. 



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

 tributaries) and the lymphatic vessels or lacteals. In general terms, 

 the proteins and carbohydrates are absorbed by the blood-vessels, 

 and the fats by the lacteals. 



Diffusion and osmosis do occur in the intestine, for if a strong 

 solution of salt is introduced into a loop of intestine, there is a flow 

 of water into the loop, owing to the high osmotic pressure of the salt ; 

 at the same time some of the salt diffuses into the blood in accordance 

 with the laws of diffusion. But if some of the animal's own serum 

 is introduced into the loop, it also is absorbed, although it has the 



* The superficial area of the small intestine, if it was flat, is about To square 

 metres. This by the presence of the villi is increased to about 42 square metres. 



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