FOOD 49 



Food, if it is to be of any use to the organism, must 

 be suited to its needs, and also in a state of solution. 

 Now every transformation of food by a third body 

 from an insoluble to a soluble and from an indiffusible 

 to a diffusible condition, whatever the precise chemical 

 change, is summed up in the term digestion. This 

 process of digestion is the same in plant and animal ; 

 it affects the same kinds of substances, produces the 

 same results, and is carried on by the same sorts of 

 agents. 



Let us now examine two prominent types of digestion 

 that of a higher mammal, say man, and that of one 

 of the higher plants. 



In animals the food, which is of three chief kinds, 

 proteids, carbohydrates, and fats, is chewed or masti- 

 cated in the mouth and mixed with the secretion of the 

 salivary gland an enzyme known as ptyalin, which 

 acts upon the starchy matter and changes it into malt 

 sugar. The mixed food is then transferred to the 

 stomach, where further secretions from the glands in 

 the stomach walls are poured into it. The chief con- 

 stituent of the secretion of the gastric glands is pepsin, 

 and by it the proteids are attacked. After a certain 

 period the food now passes into the intestine, where it 

 receives the secretions of the intestinal glands, one of 

 which is specially active in changing cane sugar into 

 glucose and fructose. Moreover, certain glands the 

 liver and the pancreas, connected with the intestine by 

 ducts, pour their secretions in. In the secretion of the 

 pancreas there is one ferment which is active in changing 

 such starchy elements as have not been touched by 

 the ptyalin, a second which attacks cane sugar, another 



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