Digestion 



17 



The foods and feeds of the preceding chapter contain the vitamins 

 and minerals discussed earlier. Along with these quantitatively minor 

 constituents, the feeds supply carbohydrates, lipides, and proteins. 

 Of these last most of the individual components recjuire chemical 

 modification before they can be utilized by animals. Thus foods are 

 subjected by the higher animals to mechanical alteration in making 

 the polysaccharides, lipides, and proteins more susceptible to hydro- 

 lytic attack. A combination of grinding, hydrolysis, emulsification, 

 and absorption is termed digestion. 



Man has developed aids to digestion in the form of preliminary 

 subdivision of the natural foodstuffs, thus decreasing both the need 

 for chewing and the period of retention of food in the stomach. In 

 addition, foods may be cooked, thereby partly hydrolyzing starches 

 and proteins. At the same time the latter are denatured, making 

 many of them more quickly sensitive to the proteolytic enzymes. 

 Finally, partial hydrolysis by acids or enzymes added to the food is 

 sometimes practiced deliberately as in tenderizing meat by such means. 



By the early part of the seventeenth century it had been suggested 

 that digestion was analogous to fermentation. Shortly afterwards it 

 was proposed instead that the muscular stomach ground the foods 

 into particles small enough for direct utilization. However, this 

 theory was invalidated by giving food in perforated, hollow-metal 

 balls. When the experimental subject regurgitated the balls after 

 keeping them in his stomach for a time, the food was gone. In the 

 nineteenth century the general features of the early stages of digestion 

 were elucidated by studies on a soldier who had a permanent opening 



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