116 



THE FEEDING OF ANIMALS 



and finally enters one of the main blood vessels. Any 

 material, therefore taken up by the lacteals ultimately 

 reaches the blood. The capillaries all converge to a larger 

 blood vessel, known as the portal vein, which enters the 

 liver, transferring to that organ whatever material the 

 capillaries have absorbed. 



170. Manner of food absorption. The manner in 

 which the soluble food is absorbed has been explained in 



part on common physical grounds. 

 When two solutions of different densi- 

 ties, containing diffusible compounds, 

 are separated by a permeable mem- 

 brane, 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 believe. The intestinal solution, 

 the denser one, is separated from a 

 less concentrated liquid, the blood, 

 which is constantly flowing on the 

 other side of a thin dividing mem- 

 brane. Under these conditions there 

 occurs the passage into the blood of 

 certain parts of the digested food. It 

 is held that in this way water, soluble 

 mineral salts, and sugar pass directly 

 into the blood vessels, chiefly from 

 the small intestine. 



171. Changes in the walls of the intestinal tract. 

 In the absorption of peptones and fats, at least, forces are 

 encountered other than the osmotic transference of sub- 

 stances in solution, the operation of which is still more or 

 less unexplained. 



FIG. 5. Intestinal 

 villas, showing: a, 

 epithelium; b, capil- 

 laries; c, lacteal ves- 

 sels. 



