251 PHYSIOLOGY OF ALIMENTATION. 



substances into the lymph or blood stream illustrates the 

 second of the above headings. 



The natures of the chemical substances which pass in this 

 way from one cell to another or from one tissue into another 

 of necessity differ greatly from each other. Limiting our- 

 selves for the moment simply to the food ingested at an 

 ordinary mixed meal we can readily recognize the very large 

 number of different chemical compounds with which we have 

 to deal. It is possible to group all of them chemically under 

 a few headings, the proteins, the carbohydrates, the fats, 

 the salts, and water. As the problem of absorption is a 

 physico-chemical one we will regroup them in a somewhat 

 different way as the colloids, the crystalloids, and water. 



Still more difficult is it to say what forces are active in 

 bringing about the movement of these various chemical 

 substances. One of the most readily intelligible of these is, 

 perhaps, diffusion, which is nothing but an expression of 

 differences in osmotic pressure. Capillary forces must also 

 be considered in a discussion of the means by which chemical 

 substances move from place to place. Finally we can mention 

 the variable mechanical affinity (OSTWALD) of colloids for water 

 and substances dissolved in the water. The forces active 

 here are not as yet clearly understood. They may be capil- 

 lary in character or connected with the enormous surface 

 presented by colloids. At any rate, modern investigations 

 support strongly the idea that an understanding of the laws 

 underlying the absorption of water by colloids will do more 

 to give us an insight into the phenomena of absorption and 

 secretion than the laws of osmotic pressure and diffusion 

 have ever done. 



But the problem of absorption is not stated when we say 

 that we have to explain the movement of a large number of 

 different chemical substances under the influence of different 

 forces. The activity of these forces is largely altered by the 

 fact that the passage of the different chemical substances 

 from cell to cell, or from liquid to cell, or from cell back to 



