422 A TEXTBOOK OF PHYSIOLOGY 



is considerable difference of opinion as to how far it is necessary for 

 protein to be broken down before it can be absorbed. It depends to 

 a certain extent upon the nature of the protein. Thus, casein, edestin, 

 acid metaprotein, introduced directly into the small intestine, are not 

 absorbed at all; egg-albumin, serum-albumin, are absorbed slightly 

 (about 20 per cent.); and alkali metaprotein considerably (70 per 

 cent.). Undoubtedly, these proteins are considerably modified, but 

 to what degree is uncertain, before reaching the blood. Proteoses 

 and peptones appear to be readily absorbed from the small intestine, 

 but it is doubtful whether they reach the blood as such. When they 

 do, it is only in such minute quantities, difficult of detection, and out 

 of all proportion with the amount actually absorbed. It is probable 

 that the absorbed proteoses and peptones are converted into amino- 

 acids by the cells of the wall of the intestine. It is known that the 

 cells of the mucous membrane are very rich in erepsin. It has been 

 shown that proteoses and peptones disappear from the lumen of an 

 isolated loop of intestine which is perfused with defibrinated blood, 

 and that no peptones can be detected in the blood. The amino-acids 

 formed as the result of digestive processes within the lumen are 

 also absorbed. In this process it is suggested the leucocytes present 

 in the mucous membrane in some way play an important part. 



The question now arises, What becomes of these amino-acids, 

 which either pass into or are formed within the intestinal wall ? It 

 is a very complex one, and at present by no means fully eluci- 

 dated. 



Before attempting to consider the two main hypotheses, it is well 

 to grasp the general idea underlying the processes of protein meta- 

 bolism. The proteins of the food are necessary to the life of the 

 animal. Without a certain amount of protein in the diet, the animal 

 slowly but surely dies. This protein is necessary for the repair and 

 growth of tissue proteins. Two points are to be observed in this 

 connection: first, that the animal's own proteins, which require 

 building afresh, differ in constitution the protein of the muscles, for 

 instance, differs from the protein of the kidney substance; secondly, 

 the ingested proteins, from which these different proteins are to be 

 replaced, differ even more widely in constitution. This is especially 

 the case when the ingested protein is of vegetable origin. It is easily 

 understood that only a portion of such ingested proteins may be 

 of service to the animal in rebuilding its particular proteins. Such 

 portion may be considerable or inconsiderable, according to the nature 

 of the protein taken in. The process of protein digestion may be 

 compared to house-breaking, the process of protein anabolism to the 

 reconstruction of a number of new houses from the bricks of the 

 demolished houses. Certain of the building-stones are of great 

 value in rebuilding the new houses, others are of partial value, 

 others are of little or no value at all. In protein anabolism, the 

 amino-acids are such building-stones. Some are of great value 

 precious others appear to be of lesser value. 



It has been shown by experiment that an animal can live when 



