CHAPTER VII 

 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 substance and energy of the animal body. It now 

 remains for us to consider the way in which nutrition is 

 accomplished. The first step in this direction is the diges- 

 tion 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 fvmctions must be generally distributed, and so 

 their compounds, in part at least, must be brought into 

 soluble and diffusible condition, in order that they may 

 pass through the membranous lining which separates the 

 blood vessels and other vascular bodies "from the cavity 

 of the alimentary canal. 



119. Digestion vs. assimilation. — In discussing physio- 

 logical 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 accom- 

 plished in what we caU the alimentary canal, a passage 

 that begins with the mouth, includes the stomach and 

 intestines, and ends with the anus. Assimilation signifies 

 the appropriation of nutrients, after digestion, to the 

 maintenance of the vital processes and to the building 

 of flesh and bone — ^processes taking place in the tissues, 



(87) 



