CHAPTER VII. 



THE PHYSIOLOGY OF DIGESTION AND 

 ABSORPTION. 



FOODS. 



IT is evident that all the tissues of the body are continually 

 undergoing "physiological wear" that the materials of which 

 they are intrinsically composed are being changed into effete 

 matter and discharged from the system. This is a process going 

 on in the substance of every cell in the body, and obviously, for 

 these cells to continue to live and functionate, there must be a 

 continual appropriation of new matter to take the place of the 

 materials which have served their physiological purpose, and are 

 of no further value to the body. This supply of material is made 

 directly to the tissues by the blood, but lest this fluid be impover- 

 ished, it must in turn be furnished with an approximately constant 

 quantity of nutritive matter. The ultimate source of that matter 

 is in the food which we eat. However, it must pass through the 

 processes of digestion and absorption before it can be utilized 

 by the tissues. This conception of a food must be understood 

 to embrace all substances contributing, either directly or indi- 

 rectly, to body nutrition, including, therefore, the oxygen of the 

 air as well as all articles usually classed as drinks. 



An animal whose weight remains about the same must eat and 

 digest a certain quantity of food to keep up the body temperature, 

 to supply mechanical energy, and to repair the wastes which are 

 continually going on in the body. An animal which is growing 

 and increasing in weight must eat enough not only to supply the 

 demands just mentioned, but also to form the new tissue. 



The articles we eat, besides being largely insoluble, differ very 

 materially in their composition from any substances found as 



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