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 func- 

 tionate, there must be a continual appropriation of new mat- 

 ter 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 impoverished, it 

 must in turn be furnished with an approximate 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 indirectly, 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 ani- 

 mal 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. 



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