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 iave absorbed. 



170. Manner of food absorption. — ^The manner in 

 which the soluble food is absorbed has been explained in 



part on common physical grounds. 



^Vhen two solutions of different densi- 



/C3r;iV4K%A ^'^^' containing diffusible compounds, 



/ mRSiW ^ separated by a permeable mem- 



t^|BjQn^jtl 1 rane, diffusion tJirough this membrane 



Irom the denser to the lighter liquid 



ill always occur. Such a condition 



i this prevails in the intestines, we 



ay believe. The intestinal solution, 



le denser one, is separated from a 



ss concentrated liquid, the blood, 



hich is constantly flowing on the 



i;her side of a thin dividing mem- 



1 rane. Under these conditions there 



;curs 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 

 villus, _ showing: a, 

 epithelium; b, capil- 

 laries; c, lacteal ves- 

 sels. 



