208 MANUAL OF PHYSIOLOGY. 



does not at all correspond with the amount that disappears from 

 the cavity of the intestine. 



Ordinary proteids, being colloids, can only pass slowly through 

 an animal membrane, hence they are said to be changed into 

 peptones under ordinary circumstances before they are absorbed. 

 Their absorption takes place chiefly in the stomach, and is com- 

 pleted in the small intestine, as only a small quantity of albu- 

 minous substances is found in the large intestine even after an 

 excessive meat diet. The more concentrated the solutions of pep- 

 tones are the more rapidly are they absorbed, and the rate of 

 absorption is greatest at first and then by degrees diminishes. 

 The presence of alkali is also said to facilitate the absorption of 

 peptones. It is a curious fact that neither in the lacteals nor in 

 the portal blood can any quantity of peptone be found, even dur- 

 ing active proteid digestion ; so that it is impossible to trace out 

 their course as peptones, or to say by which set of channels they 

 reach the blood. If we assume that all proteids must be absorbed 

 as diffusible peptone, we are forced to conclude that during their 

 passage from the intestinal cavity they must be reconverted into 

 ordinary proteids. But we know that soluble forms of albumin 

 are to some extent diffusible (when a solution of salt is used) 

 through a dead animal membrane. But even were this quite im- 

 possible, it would not preclude the possibility of their passing 

 through the intestinal wall, which presents no such obstacle, for 

 it is a living structure that can overcome such physical difficul- 

 ties as the non-diffusion of colloids. When we know that solid 

 particles of fat can enter the lacteals, we can have no difficulty 

 in believing that a solution of albumin is admitted. We may 

 then conclude that it is not only possible, but even probable, that 

 a good deal of proteid is absorbed as ordinary soluble albumin. 

 A certain limit to proteid absorption exists, so that any albumin- 

 ous materials above the maximum that escape conversion into 

 leucin and tyrosin are thrown off with the faeces. 



In the absorption of water, watery solutions of salts, sugars, 

 and peptones, there are no physical difficulties to be got over ; so 

 that we are in the habit of speaking confidently about the mech- 

 anism of their absorption, although in all probability many cir- 



