2 55 

 CHAPTEE IX. 



DIGESTION. 



DIGESTION is the process by which those parts of our food 

 which may be employed in the formation and repair of the 

 tissues, or in the production of heat, are made fit to be 

 absorbed and added to the blood. 



Food. 



Food may be considered in its relation to these two pur- 

 poses the nutrition of the tissues, and the production of 

 heat. But, under the first of these heads will be included 

 many other allied functions, as, for example, secretion and 

 generation : and under the second, not the production of 

 heat only as such, but of all the other forces correlated 

 with it, which are manifested by the living body. 



The various articles of food may be artificially classified 

 according as they are chiefly subservient to one or the 

 other of these purposes. All articles of food that are to be 

 employed in the production of heat, contain a large amount 

 of carbon and hydrogen ; and of those which are appro- 

 priate for the maintenance of the several tissues (except 

 the adipose) nearly all are characterised by the possession 

 of nitrogen, and are capable of ready conversion into the 

 nitrogenous principles of the blood. 



The name of nutritive OT plastic is given to those principles 

 of food which admit of ready conversion into the albumen 

 of the blood, and of being subsequently assimilated, through 

 the medium of the blood, by the tissues. And those prin- 

 ciples, comprising the greater part of the non-nitrogenous 

 materials of food, in the form of fat, starch, sugar, gum, 

 and other similar substances, which are believed to be 

 employed in the production of heat, are named calorifacient, 

 or, sometimes, respiratory food. It must be borne in mind, 

 however, that the nitrogenous articles of food are in part 



