CHAPTEK III. 



DIGESTION AND DIGESTIBILITY. 



On examining any ordinary foodstuff in the dry state, it is 

 evident that many changes must take place before, from 

 this rough, dry, insoluble material, soluble nutritive 

 substances can get through the intestinal wall into the 

 blood-stream to feed all the tissues of the body. These 

 several changes constitute the process of digestion. It 

 will be easily understood that different foodstuffs will 

 require different treatment, in order that all their useful 

 food material may be extracted from them; and it is 

 common experience, alike in man and animals, that one 

 food is more digestible than another. Thus it is 

 necessary in appraising the value of any one food to 

 know to what extent the nutritive substances it contains 

 are digested, for it is not the amount of food ingested, 

 but the amount digested, which really gives nutriment to 

 the animal. 



When a food is taken into the mouth, it is first masti- 

 cated or broken up by the grinding teeth, mixed with 

 saliva, and then swallowed. Mastication is important in 

 that it allows thorough penetration of the digestive juices 

 to all parts of the food, and especially is this necessary in 

 the case of grains covered with a fibrous envelope or husk, 

 itself but little digested. In the mouth, then, the food- 

 mass is ground down into small particles and saturated 



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