544 THE ABSORPTION OF FOOD [CH. XXXVI. 



same osmotic pressure and concentration as the animal's blood. This 

 experiment alone shows us that known physical laws will not com- 

 pletely explain absorption. In fact, absorption is a subject upon 

 which we can speak with little certainty ; the energy that controls 

 it is doubtless some form of imbibition, and resides in the living 

 epithelium ; for if the epithelium is injured or destroyed by the 

 action of such a poison as sodium fluoride, absorption almost ceases, 

 and what does occur follows the laws of osmosis and diffusion. 



A marked feature during absorption is the increased activity of 

 the lymphocytes which lie beneath the epithelium ; the number of 

 these cells in the blood increases markedly ; it may be even doubled. 

 It has, therefore, been surmised that these cells share in the work of 

 transporting absorbed materials. 



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 

 passes. Cane sugar and milk sugar are also converted into mono- 

 saccharides before absorption. 



The carbohydrate food which enters the blood as glucose is taken 

 to the liver, and there stored up in the form of glycogen a reserve 

 store of carbohydrate material for the future needs of the body. 

 Glycogen, however, is found in animals who take no carbohydrate 

 food. It must, then, be formed by the protoplasmic activity of the 

 liver cells from their protein constituents (see preceding chapter). 

 Monosaccharides (and especially glucose) are the only sugars from which 

 the liver is capable of forming glycogen. If other carbohydrates such 

 as cane sugar or lactose are injected into the blood-stream direct, they 

 are unaltered by the liver, and finally leave the body by the urine. 



Absorption of Proteins. It is possible for the alimentary canal 

 to absorb soluble protein in an unchanged condition. Thus, after 

 taking a large number of eggs, egg-albumin is found in the urine. 

 Patients fed per rectum derive some nourishment from protein food, 

 although proteolytic enzymes are absent from that part of the 

 intestine. But such occurrences are. exceptional ; they are merely 

 illustrations of the fact that under unusual conditions certain parts 

 of the body can rise to the occasion and perform unusual feats. 



The normal course of events is that the food proteins are broken 

 up into their constituent amino-acids, and it is in this form that 

 they are absorbed. If an animal receives, instead of protein the 

 final cleavage ^ products of pancreatic digestion, it continues to 

 maintain its nitrogenous equilibrium; that is to say, the cells of 

 the body are able to synthesise tissue-proteins from the fragments of 

 the food proteins. 



