MECHANISM OF ABSORPTION 425 



characteristic thing about adsorption is that at the equilibrium-point 

 the concentration of the dissolved substance (or the gasj on the surface 

 is immensely greater than in the general mass of the solution. The con- 

 centration on the surface can, indeed, be increased by increasing the 

 concentration of the solution, but in a far smaller proportion. Accord- 

 ing to the thermodynamic law enunciated by Willard Gibbs, sub- 

 stances which diminish the surface tension must tend to accumulate 

 at the surface, and substances which increase the surface tension must 

 tend to diminish in concentration at the surface. If a small quantity 

 of a substance diminishes the surface tension at a given surface more 

 in proportion than a larger quantity, not only will there be an accumula- 

 tion of the substance at the surface, but this will be proportionally 

 greater for small than for larger concentrations of the substance in the 

 solution. This characteristic feature of adsorption may thus depend 

 entirely on surface forces due to the conditions under which the attrac- 

 tion of the molecules for each other acts at the surface. It has not been 

 shown, however, that chemical forces due to the interaction of the 

 electrically charged ions are not also concerned. What is especially 

 important to point out is that in the tissues of the body there is a 

 great development of surfaces. The cell walls or cell envelopes come 

 into contact with the tissue lymph or the contents of the digestive 

 tube or the secretions in the alveoli of glands on the one side, and the 

 cell contents on the other, and constitute in the aggregate an immense 

 surface. The surfaces separating the nuclei from the cytoplasm, and, 

 above all, the surfaces of the particles of the contents of cytoplasm 

 and nuclei suspended in colloid solution, offer prodigious opportunities 

 for such surface phenomena as adsorption. 



SECTION II. MECHANISM OF ABSORPTION. 



In the preceding chapter we have traced the food in its progress 

 along the alimentary canal, and sketched the changes wrought in 

 it by digestion. We have next to consider the manner in which it 

 is absorbed. Then, for a reason which has already been explained, 

 instead of following its fate within the tissues, until it is once more 

 cast out of the body in the form of waste products, it will be best 

 to drop the logical order and pick up the other end of the clue in 

 other words, to pass from absorption to excretion, from the first 

 step in metabolism to the closing act, and afterwards to return and 

 fill in the interval as best we can. 



Comparative. And here, first of all, it should be remembered that 

 the epithelial surfaces, through which the substances needed by the 

 organism enter it, and waste products leave it, are, physiologically con- 

 sidered, outside the bodv. The mucous membranes of the alimentary, 

 respiratory, and urinary tracts are in a sense as much external as the 

 fourth great division of the physiological surface, the skin. The two 

 latter surfaces ai^ in the mammal purely excretory. Absorption is 

 the dominant function of the alimentary mucous membrane, but a 

 certain amount of excretion also goes on through it. The pulmonary 

 surface both excretes and absorbs, and that in an equal measure. But 

 it is by no means necessary that the surface through which oxygen is 

 taken in and gaseous waste products given off should be buried deep in 

 the body, and communicate only by a narrow channel with the exterior. 



