ABSORPTION OF CARBOHYDRATES AND PROTEIDS. 431 



THE ABSORPTION OF CARBOHYDRATES AND PROTEIDS. 



It was for many years believed that the absorption of the products of 

 digestion from the alimentary canal was governed by exactly the same 

 physical laws as determine the passage of a solution and its dissolved 

 constituents through an inert membrane, but the accumulation of 

 experimental evidence has rendered such a belief no longer tenable. 

 It is now known that the cells which line the alimentary canal take an 

 active part, not only in absorbing the materials prepared for them by 

 the action of the digestive secretions, but in modifying these products 

 in various ways during the process. 



Before the laws of diffusion of solutions were known, the process of 

 absorption by the columnar cells of the intestine was compared by Tiedemann 

 and Gmelin (1820) l to that of gland secretion. After the establishment of the 

 laws of diffusion, attempts were made to apply them in explanation of absorp- 

 tion, as well as of other similar processes in the body. Such physical views 

 persisted for a long time, until it was shown by conclusive experiments that 

 absorption^ like these other processes, does not obey the laws of physical 

 diffusion, but is selective in its character and governed in some subtle way 

 by the activity of the cells involved. Our modern view is thus, as is often 

 the case, a recurrence to an older theory ; the only difference being that we 

 have a somewhat broader experimental basis on which to build it. 



The cells of a secreting gland take up certain materials from the lymph in 

 which they are bathed, and from these, in some manner, elaborate certain 

 products which are passed into the gland lumen as a secretion. Similarly, the 

 absorbing cells of the intestine take up certain products of digestion from 

 the intestinal contents by which they are bathed, and build up from these 

 certain materials which pass into the lymph. So that absorption may be 

 regarded as a kind of reversed secretion. 



In both cases the process is a selective one, the constituents of the gland 

 secretion are definite in their nature, in many cases specific, and are probably 

 formed from definite constituents of the lymph taken up by the secreting cell 

 to the exclusion of others. In like fashion, certain materials only are taken 

 up by the epithelial absorbing cell, and from these definite products are 

 formed to be passed into the lymph. 



That absorption is a selective process and not one of purely physical 

 diffusion, is shown by the following observations : 



1. Certain colloids (e.g. alkali albumin) disappear from the intestine 

 at a fairly rapid rate, even in the complete absence of digestive 

 enzymes. 2 



2. The rate of absorption from the intestine of various dissolved 

 substances is not proportional to their diffusion-coefficients. Sodium 

 sulphate is much more diffusible than grape-sugar, but when a solution 

 containing 0-5 per cent, of each of these is injected into the intestine, 

 the sugar disappears much more rapidly, and only traces of it remain 

 at a time when the greater part of the sodium sulphate is still left 

 behind. 3 



3. The rapidity of absorption is much greater than can be accounted 



1 Quoted by Heidenhain, Arch. /. d. ges. PhysioL, Bonn, 1888, Supp. Heft, Bd. 

 xliii. S. 69. 



2 See form of absorption of proteids, p. 436. 



3 Rohmann, Arch. f. d. ges. PhysioL, Bonn, 1887, Bd. xli. S. 411. 



