120 The Feeding of Animals 



The manner in which the soluble food is absorbed 

 may be explained in part on common physical grounds. 

 When two solutions of different densities, containing 

 diffusible compounds, are separated by a permeable 

 membrane, diffusion through this membrane from the 

 denser to the lighter liquid will always occur. Such a 

 condition as this prevails in the intestines, we may be- 

 lieve. The intestinal solution, the denser one, is sep- 

 arated from a less concentrated liquid, the blood, which 

 is constantly flowing on the other side of a thin dividing 

 membrane. Under these conditions only one thing can 

 occur; viz., the passage into the blood of certain parts 

 of the digested food. It is held that in this way w^ater, 

 soluble mineral salts and sugar pass directly into the 

 blood-vessels. The peptones are taken np largely by 

 lacteals and the fats enter the blood entirely' through 

 this channel. 



In the absorption of peptones, ^^e encounter forces 

 other than those which pertain to the mere diffusion of 

 liquids, the operation of which is still more or less 

 shrouded in mystery. As we have learned, the proteids 

 are largely changed to peptone in the stomach and in- 

 testines, but, strange as it may seem, no peptone is 

 found in the blood. At some point in its passage 

 through the lining tissues of the digestive tract, it has 

 been regenerated into forms more nearly like those from 

 which it is derived. Moreover, the absorption of fats 

 is regarded as being accomplished through the activity 

 of certain cells or corpuscles, which appear to convey 

 tliis portion of the food to the lacteals. It seems, then, 

 that the vital forces residing in the living animal cells 



