DIGESTION. ;( 



PHYSIOLOGY. 



ns 



We are not obliged to eat all these food-stuffs. 

 Proteid matter we must have always. It is the only 

 food-stuff which contains nitrogen. It is the only 

 substance which can renew the nitrogenous proteid 

 matter of the blood and so the nitrogenous proteid 

 matter of the body. 



We might indeed manage to live on proteid matter 

 alone, for it contains not only nitrogen but also carbon 

 and hydrogen, and out of it, with the help of a few 

 minerals, we might renew the whole blood and build 

 up any and every part of the body. But, as you will 

 V Ti hereafter, it would be uneconomical and unwise 

 tv do so. Starch, sugar, and fats, contain carbon and 

 hydrogen without nitrogen ; and hence, if we are to 

 live on these we must add some proteid matter to 

 them. 



50. Of these food-stuffs, putting on one side the mine- 

 rals, sugar (of which, as you know, there are several 

 kinds, cane sugar, grape sugar, and the like) is the only 

 one which is really soluble, and will pass readily by os- 

 nosis through thin membranes (see p. 84). If you take 

 a quantity of white of tgg^ cr blood serum, or meat, or 

 fibrin,' or a quantity of starch boiled or unboiled, or a 

 quantity of oil or fat, place it in a bladder, and im- 

 merse the bladder in pure water, you will find that none 

 of it passes through the bladder into the water outside, 

 as sugar or salt would do. In the same way a quantity 

 of meat, or of starch, or of fat, placed in your alimen- 

 tary cp" al, would never get through the membrane 

 which separates the inside of the canal from the inside 

 of the capillaries, and so would remain perfectly useless 

 as food unless something were done to it. While the 

 food is simply inside tae alimentary canal, it is really 



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