CHAPTER VIII 



THE DIGESTION OF FOOD 



WE have accepted so far without discussion the 

 almost self-evident fact that the food is the immediate 

 source of the energy and substance of the animal body. 

 It now remains for us to consider the way in which the 

 nutrition of an animal is accomplished. The first step 

 in this direction is the digestion of food. It is necessary 

 for food ingredients to be placed in such relations to the 

 animal organism that they are available for use. This 

 involves both condition and location. The various 

 nutrients in the exercise of their several functions must 

 be generally distributed in all the interior parts of the 

 animal. It is obvious that hay and grain as such cannot 

 be so distributed, and so their compounds must, in part 

 at least, be brought into a soluble and diffusible condi- 

 tion, in order that they may pass through the mem- 

 branous lining which separates the blood-vessels and 

 other vascular bodies from the cavity of the alimentary 

 canal. 



In discussing physiological relations of food, two 

 terms are employed: viz., digestion and assimilation. 

 Digestion refers to the preparation of food compounds 

 for use, by rendering them soluble and diffusible, 

 changes which are accomplished in what we call the ali- 



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