114 



ADVANCED PHYSIOLOGY 



OSMOSIS 



How is food actually absorbed through the intestinal walls? 

 Two liquids, the blood and the dissolved food, are separated 



by a thin moistened membrane. 

 How is it possible for the liquid 

 to flow in one direction and not in 

 the other? In other words, why 

 does the liquid food go through 

 into the blood vessels, and the 

 blood at the same time not pass 

 out into the intestine? Physiol- 

 ogists are as yet able to give only 

 a partial answer to this rather 

 puzzling question. 



Food absorption depends partly 

 upon a process called osmosis. 

 In the first place it is not quite 

 true that food passes from the 

 intestine, and that nothing passes 

 in the other direction; for a cer- 

 tain amount of liquid does pass 

 into the intestinal canal from 

 the blood. But the latter is 

 small in amount, perhaps chiefly 

 water, and more material passes 

 in the reverse direction. To 

 show how such a transfer could 

 take place, an illustration will be 

 useful. Procure a piece of mem- 

 branous tube, like that used for 

 the covering of sausages. Fill 



this with a solution of pure grape sugar and suspend it in 

 a jar full of water, as shown in Figure 63. The tube may 

 thus represent the intestine full of digested food, and the 



FIG. 62. SHOWING THE MI- 

 NUTE BLOOD VESSELS OF 

 THE VILLI AND INTESTINAL 

 WALLS 



The arteries are striped, the veins 

 black and the capillaries open. 

 (Oppel) 



