1 62 ORGANIC AGRICULTURAL CHEMISTRY 



harder the solid food the more thorough must be the mastica- 

 tion. This is why the food of herbivorous animals like the 

 horse and cow must be subject to the most thorough mastica- 

 tion, and in the latter animal and other ruminants is subject 

 to a second mastication. Mastication in all the common 

 animals, except the fowls, takes place in the mouth by means 

 of the teeth. Thus we find that animals which naturally eat 

 the tougher foods, like raw plants, have the more elaborate 

 and stronger masticating teeth. 



Digestion. — The animal body is composed of material 

 more or less saturated with water. All of the body fluids are 

 water solutions or suspensions and the cells contain water 

 solutions. The cell walls are membranes permeable to water 

 and semipermeable to substances in solution. A living cell 

 which is thoroughly dried becomes dead as a consequence. 

 The whole body may thus be considered as a water system. 

 Thus solubility in water and permeability through cellular 

 membranes are necessary conditions for food material that is to 

 be absorbed and built up into body substance. Of the three 

 organic food constituents the carbohydrates are the only ones 

 that are soluble in water to any extent. Even of the carbo- 

 hydrates the most widely distributed member considered as a 

 food is starch which is insoluble. The fats are wholly in- 

 soluble and of the proteins only one group, viz. the albumins, 

 is soluble in water. The conversion of the insoluble forms of 

 food material into soluble is thus essential and is brought about 

 by the process of digestion. 



Not only, however, must the food material be soluble, but 

 it must be brought into certain definite forms so that the sub- 

 sequent changes of metabolism may take place. This may be 

 illustrated by the fact that though cane sugar and milk sugar 

 are soluble compounds they, nevertheless, undergo digestive 

 changes, because in their unchanged character they cannot be 

 built up into body substance or oxidized. 



We may, therefore, say that the object of digestion is two- 

 fold: first, to convert insoluble food materials into soluble com- 



