CHAPTER VII 



DIGESTION 



THE purpose of digestion is to so change the foodstuffs that they can pass 

 into the blood and be utilized in metabolism. To this end the food is sub- 

 jected to mechanical division and chemical transformation in the digestive 

 organs. 



Of all the combustible constituents of our diet, only sugar is soluble in 

 water. Starch and coagulated proteid are insoluble in water ; but by digestion 

 they are so changed that they can be taken into solution. Fat also, which is 

 insoluble in water, is transformed so that it can be absorbed from the ali- 

 mentary tract into the blood. 



The organic foodstuffs, which are already soluble, undergo transformations 

 in the alimentary canal which adapt them to the requirements of metabolism. 

 The noncombustible constituents of the diet, water and the salts, do not require 

 to be digested in order to be taken into the blood. 



In man the work of the digestive system is materially aided by the prepa- 

 ration of " dishes " of food, since the foodstuffs are thereby rendered more 

 easily accessible to the digestive fluids. 



The heat necessary to boil or roast meat swells the connective tissue, which 

 holds the muscle fibers together, and changes it partly into gelatin. The meat 

 at the same time becomes less compact, and so is the more readily reduced to 

 fine bits by our teeth. In the cooking of vegetable foods, the cell membranes 

 are ruptured by the heat and the starch granules are transformed into a soluble 

 modification. In bread baking the dough is rendered spongy by the carbon 

 dioxide formed in " raising," and this is carried still further by the heat of the 

 oven, by which also the starch granules are transformed in the same way as in 

 ordinary cooking. 



FIRST SECTION 



THE DIGESTIVE FLUIDS 



1. GENERAL SURVEY 



The fluids secreted by the digestive glands serve either to change the 



chemical nature of the foodstuffs so that they shall be fit for absorption, or 



they aid the processes going on in the alimentary canal in some other way. 



Certain products also which are given off with the digestive fluids are des- 



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