CHAPTER XIII* 

 HOW ANIMALS DIGEST 



The Object of the Digestive Process is the reduction 

 of food into such a state that it can be absorbed into the 

 system. For this purpose, if solid, it is dissolved ; for 

 fluidity is a primary condition, but not the only one. 

 Many soluble substances have to undergo a chemical 

 change before they can form parts of the living body. 

 If albumen or sugar be injected into the veins, it will 

 not be assimilated, but be cast out unaltered. 



To produce these two essential changes, solution and 

 transmutation, two agencies are used one mechanical, 

 the other chemical. The former is not always needed, 

 for many animals find their food already dissolved, as 

 the butterfly ; but solid substances, to facilitate their 

 solution, are ground or torn into pieces by teeth, as in 

 man ; by jaws, as in the lobster; or by a gizzard, as in 

 the turkey. 



The chemical preparation of food is indispensable. 101 

 It is accomplished by one or more solvent fluids secreted 

 in the alimentary canal. The most important, and one 

 always present, is the gastric juice, the secretion of 

 which is restricted to the stomach, when that cavity 

 exists. In the higher animals, numerous glands pour 

 additional fluids into the digestive tube, as saliva into 

 the upper part or mouth, and bile and pancreatic juice 

 into the upper part of the intestine. In fact, the mucous 



* See Appendix. 

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