658 THE MECHANISM OF ABSORPTION 



solution and the less aggregation proceeding until the molecules 

 could diffuse out of the cell again. 



Absorption of Proteid. We may form a working hypothesis 

 of this process which would begin as follows. Proteids must be 

 digested before absorption, that is, they must be dissolved and 

 undergo hydrolytic cleavage, at least as far as the albumose stage. 

 Albumoses are diffusible through parchment paper, and we may 

 believe that they might diffuse into the epithelial cells, undergo 

 change there, and so keep up absorption. But when we have got 

 as far as this we are faced by the difficulty that we have no 

 certain knowledge of what change proteids undergo in the epi- 

 thelial cells. If the view of Folin is right, that the major part of 

 the absolved proteid is immediately broken down further, we can 

 still proceed. For we may believe that albumose is further hydro- 

 lysed by erepsin and split into amido-acids, that the NH 2 groups 

 of these acids are split off, and that the ammonia and non- 

 nitrogenous moieties pass into the portal blood by diffusion. 

 There can be little doubt that Folin' s view explains the known 

 facts, but it is doubtful where these changes actually take place, 

 whether in the lumen of the gut or in its mucous membrane. 

 If, on the other hand, we believe that any of the absorbed 

 albumose undergoes aggregation within the cell and is there 

 built up into a complex proteid, then the passage of such a 

 substance into the blood becomes difficult to explain by a 

 physical process, unless we believe that the cell on its blood side 

 and the capillary wall are permeable to proteids as complex as 

 the blood proteids. 



Absorption of CaboTiydrates. The chief end product of starch 

 digestive appears to be maltose. We may imagine that this 

 diffuses into the epithelial cell and becomes converted by maltose 

 invertin into dextrose, which in turn passes by diffusion into the 

 blood. As in the case of fat and proteid absorption, it is neces- 

 sary to believe that the new product formed in the cell cannot 

 diffuse back into the gut. Lactose and cane-sugar similarly are 

 changed by ferments during absorption. We have seen that a 

 solution of dextrose is absorbed as rapidly as one of maltose, and 

 yet dextrose undergoes no change by ferments during absorption. 

 We have not the data which enable us to discuss how far the 

 absorption of dextrose could be a physical process. We are un- 

 certain how far dextrose in the portal blood is in a free con- 



