116 ADVANCED PHYSIOLOGY 



Both sugar and water molecules will pass through, but the 

 sugar molecules, since they are larger, will not pene- 

 trate as fast as water. Hence, there will be much 

 more water going into the tube than sugar going out. 

 Nevertheless, if we leave the tube in the jar long enough the 

 sugar will continue to pass out till the solution in the jar 

 becomes as sweet as that in the tube. After this, no noticeable 

 exchange occurs. But if we should then remove the water, 

 which by this time would contain some of the sugar, 

 and replace it with fresh water, once more putting the 

 tube in it, the sugar would then continue passing out. If we 

 continued to renew the water in the jar as fast as it became 

 charged with the sugar we could keep the sugar flowing 

 out of the tube into the water of the jar until all of 

 the sugar was gone from the tube, and only pure water 

 was left in it. 



Food Absorption. Now, this process of osmosis does not 

 fully explain the manner of food absorption, but it does ex- 

 plain certain phases of it. Physicists find that some sub- 

 stances will thus pass through membranes while others will 

 not. The former they call crystalloids, the latter colloids. 

 Most of our foods, when eaten, are of such a character that 

 they will not pass through membranes, and could not be 

 taken through the intestinal walls; but digestion changes 

 them, until finally they are in a form that will readily diffuse. 

 Starches and proteids, for example, will not diffuse, while 

 sugars and peptones will. Thus, digestion brings the food 

 into a condition in which it can be absorbed. 



In the intestine digested food is on one side of the mem- 

 brane formed by the epithelium of the villi, while on the other 

 side is blood; the membrane thus has different liquids moisten- 

 ing its two sides. Under these conditions the dissolved food 

 begins to flow through into the blood vessels, and as fast as 

 the blood present becomes filled with the absorbed food, it is 

 carried off and fresh blood takes its place. This continues 



