CHAPTER III 

 THE NATURE AND THE MEANS OF DIGESTION 



IT has been said that one of the results of the specializa- 

 tion of cells is the loss of the primitive power to receive 

 and utilize all kinds of food. The blood brings to the tis- 

 sues of the body food of certain standard forms, and it is 

 only these which can be used. The attempt to add various 

 soluble foods to the blood by direct injection into the 

 circulation has shown that in most cases such foods are 

 offered in vain to the living cells. Milk introduced in 

 this way is not a practical means of nutrition. Cane-sugar 

 added in measured amounts to the blood is excreted 

 promptly and in almost undiminished quantity by the 

 kidneys. Thus it becomes clear that foods introduced 

 directly into the blood are frequently treated like waste 

 products, while the same foods after transformation in the 

 alimentary canal are entirely acceptable to the body cells. 

 The function of the alimentary canal is to work over the 

 many foreign forms of nutriment into a few forms of the 

 native type. From day to day the diet may be of quite 

 variable character, but its variations hardly show them- 

 selves in the composition of the blood. 



The term digestion is usually applied to those changes in 

 the food-stuffs which precede absorption. To cover sub- 

 sequent changes we use the word_metabolism. 



One of the more evident characteristics of digestion is 

 that it is a refining process. It effects a separation of the 

 useful and the useless portions of the ration. This is 

 a very conspicuous fact with the herbivora, whose food 

 contains much woody material from which the available 

 nutriment must be laboriously extracted. With man- 

 kind, especially under modern conditions, a great part of 



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