THE STORY OF THE LIVING MACHINE. 



...C 



intestinal contents holds more dialyzable material 

 than the blood. Under these conditions, which 



will always occur 

 after food has been 

 digested by the di- 

 gestive juices, the 

 food will begin to 

 pass through this 

 membranous wall of 

 the intestine into the 

 blood under the in- 

 fluence of the physi- 

 cal force of osmosis. 

 Thus the primary 

 factor in food absorp- 

 tion is a physical one. 

 We must notice, 

 however, that the 

 physical force of os- 

 mosis is not the only 

 factor concerned in 

 absorption. In the 

 first place, it is found 

 that the food during its passage through the intes- 

 tinal wall, or shortly afterwards, undergoes a further 

 change, so that by the time it has fairly reached 

 the blood it has again changed its chemical nature. 

 These changes are, however, of a chemical nature, 

 and, while we do not yet know very much about 

 them, they are of the same sort as those of diges- 

 tion, and involve probably nothing more than 

 chemical processes. 



Secondly, we notice that there is one phase of 

 absorption which is still obscure. Part of the food 

 is composed of fat, and this fat, as the result of di- 

 gestion, is mechanically broken up into extremely 



FIG. 3. Diagram of the intestinal 

 walls. /4, lumen of intestine 

 filled with digested food. B, 

 villi, containing blood-vessels. 

 C, larger blood vessel, which 

 carries blood with absorbed food 

 away from the intestine. 



