ALIMENTARY TRACT AS AN ^ABSORPTIVE SYSTEM. 251 



liquid, is modified by the existence of differences in the perme- 

 ability of protoplasm, more particularly by the existence of 

 cell membranes which are sometimes permeable, sometimes 

 impermeable, or again partially permeable to a diffusing 

 substance. Not only do different cells differ in their perme- 

 ability to certain chemical substances, but the same cell may 

 under different conditions be at times permeable, at others 

 impermeable, to the same substance. 



When we consider that under ordinary circumstances the 

 different chemical substances, the different forces, and the 

 different permeabilities of protoplasm are all working to- 

 gether in making up a picture of absorption as we witness it 

 in a physiological experiment some idea may be obtained 

 of the difficulties which face the investigator who attempts 

 the solution of the problem. Nevertheless great strides have 

 been made within recent years in substituting known laws of 

 physics and chemistry for the vitalistic explanations of the 

 older observers. In the following paragraphs are discussed 

 in brief the physical chemistry of the substances which serve 

 as food, the forces active in bringing about their absorption, 

 and the membranes which alter so markedly the independent 

 activities of the other two. It is beyond the scope of this 

 volume to enter more deeply than this into the subject*. 



2. The Physical Character of the Foodstuffs. Colloids 

 and Crystalloids. From the standpoint of absorption the 

 chemical constitution of the foods which we consume plays 

 less of a role than their physical character. It is for this 

 reason that a regrouping of these substances into colloids, 

 , crystalloids, and water has been suggested, for, as we shall 

 see, the readiness with which they diffuse, for example, is 

 of greater import than the arrangement of the atoms which 

 constitute their molecules. 



As many as fifty years ago GRAHAM recognized that dif- 

 ferent chemical substances differ greatly in the rate with 

 which they diffuse through solvents of various kinds. Those 

 which diffuse very slowly aro for the most part amorphous, 



