ABSORPTION OF CARBOHYDRATES AND PROTEIDS. 43 . 
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. f. 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. 
