254 PHYSIOLOGY OF ALIMENTATION. 



is incapable of doing so. Moreover, certain colloids may be 

 obtained artificially, not only in an amorphous state but 

 also in a most beautifully crystalline form (HOFMEISTER). 



So far as absorption is concerned, we can say that nearly 

 every food which enters the alimentary tract belongs in one 

 or the other of these two groups, or does so after it is acted 

 upon by the digestive juices. As examples of the colloids 

 we may mention all the different albumins, globulins, albu- 

 minoids, and starch paste; under the crystalloids, the vari- 

 ous sugars, salts, acids, and alkalies. We begin to see now 

 the importance of the various digestive processes which go 

 on in the intestine. While it will be shown later that albu- 

 mins, for instance, may perhaps be absorbed as such, possibly 

 are even absorbed in part in an unaltered state, it is self 

 evident that as these colloids become more like crystalloids 

 their diffusibility, and in consequence their absorption, will 

 be greatly facilitated. In the process of digestion this trans- 

 ference from the side of the non-diffusible colloids to that of 

 the diffusible crystalloids does in fact occur. The peptones are 

 much less colloidal in character than the albumins from which 

 they come, and the ultimate products of digestion formed 

 under the influence of the proteinases or protease are prac- 

 tically all typical crystalloids. Starch paste also leaves the 

 side of the colloids when acted upon by amylase and as mal- 

 tose becomes grouped with the crystalloids. The fats which 

 as such are incapable of diffusion diffuse readily after having 

 been acted upon by lipase and changed into fatty acid and 

 glycerine. 



3. Membranes. We shall consider next not the forces that 

 bring about absorption, which would seem most logical, but 

 rather the obstacles which modify absorption namely, mem- 

 branes of all kinds. This will make what is to follow more 

 intelligible. 



As we are interested in membranes chiefly from the stanc 

 point of whether they allow substances to diffuse through 

 them or not and to what extent they permit this, the classifi- 



