Food and its Digestion. 



253 



But with man and animals it is different, they cannot sub- 

 sist on the inorganic substances alone, but need for support 

 materials which have already been organized, and which 

 have previously formed a constituent part of animal or 

 vegetable bodies. 



In animals the food required must undergo a process 

 of digestion or liquefaction, before it can be absorbed, the 

 general characteristics of which in all cases are the same. 



Digestion is accomplished in the alimentary canal, 

 where the food is brought under the action of certain 

 digestive fluids, which liquefy and dissolve it. These 

 fluids are the secretions from the mucous membrane of 

 the alimentary canal, and those glandular organs conti- 

 guous, which pour their seoretions into this canal. 



Because the food, as we have seen, always consisting of 

 a mixture of substances having different physical and 

 chemical properties, the digestive fluids necessarily must 

 differ materially from each other, each one exerting a 

 peculiar action, which is more or less limited to particular 

 species of food. 



The different elements of the food are digested iu dif- 

 ferent parts of the alimentary canal, by the agency of 

 these different digestive fluids, and by their action the 

 various ingredients of the alimentary mass, are successively 

 reduced to a fluid condition, so as to be taken up by the 

 blood vessels, and the absorbent vessels of the intestinal 

 mucous membrane, thus to be introduced into the general 

 circulation. 



The action which is exerted upon the food by the 

 digestive fluids, is by no means that of a simple chemical 

 solution. It is an actual transformation, by means of 

 which the ingredients of the food are altered in character, 

 at the same moment that they undergo the process of 

 liquefaction. 



The active agent in producing this change, in every 

 instance, is an organic principle entering as an ingredient 

 into the digestive fluid, and which by coming into contact 



[Trans. 33 



