120 GENERAL ZOOLOGY 



are ready to pass with any inorganic foods — as, for ex- 

 ample, salt and water — through the wall of the intestine. 

 The enzymes just discussed are very important and it is 

 only by their activity that many foods are changed in the 

 digestive processes. A considerable number of other enzymes 

 and materials produced by the liver and by glands in the 

 walls of the stomach and intestine aid in digestion. 



Absorption. Digested food passes through the intestine 

 wall and mixes with the body fluids, in accordance with a 

 principle known in physics as osmosis. It is well known 

 that a crystalloid, like salt or sugar, if placed in a vessel of 

 water, will soon diffuse through the water. In doing so it 

 exerts a certain amount of pressure. This is called osmotic 

 pressure. If a permeable membrane be interposed between 

 a solution of crystalloids and one of colloids, both sub- 

 stances will exert some osmotic pressure, but by far the 

 greater amount is exerted by the crystalloid, because of its 

 greater facility in diffusing through water. 



Applying the principle of osmosis to absorption in the in- 

 testine, we have crystalloids in the cavity of the intestine, 

 a permeable though not porous membrane (the intestine 

 wall), and colloids in the blood and body-cavity fluid. The 

 digested food diffuses through the intestine wall because of 

 the pressure it exerts in seeking to mix with more water. 

 Food on passing to the blood is changed by the cells of the 

 intestine wall from the crystalloid condition to a colloid, 

 and is henceforth incapable of passing back through the 

 membrane. 



Assimilation. The food is transported by the blood in the 

 blood vessels or in the body cavity to tissues, where some of 

 it is transformed into protoplasm. Just how this is done no 

 one has been able to discover, but it is known that the trans- 

 forming process, which is called assimilation, takes place 

 in all tissues that are alive, — for example, muscles and 



