256 DIGESTION. 



devoted to the maintenance of the heat of the body ; and 

 it is equally true that the non-nitrogenous or so-called 

 calorifacient food has to do in part with nutrition ; so that 

 care must be taken not to rely implicitly on the name 

 which has been given to these two classes of nutriment. 

 For purposes of accurate classification, the terms may be 

 best abandoned altogether. 



The following is a convenient tabular classification of 

 the usual and more necessary kinds of food : 



NITROGENOUS : 



Albumen, Casein, Gluten, Gelatin, and their allies (containing Carbon, 

 Hydrogen, Oxygen, and Nitrogen ; some of them, also Sulphur and 

 Phosphorus). 



NON- NITROGENOUS : 



(i). Starch, Sugar, Alcohol, and their allies (containing Carbon, 

 Hydrogen and Oxygen). 



(2). Oils and Fats (containing Carbon, Hydrogen, and Oxygen; the 

 oxygen in much smaller proportion than in starch or sugar). 



(3). Mineral or Saline Matters ; as Chloride of Sodium, Phosphate of 

 Lime, etc. 



(4). Water. 



Animals cannot subsist on any but organic substances, 

 and these must contain the several elements and com- 

 pounds which are naturally combined with them : in other 

 words, not even organic compounds are nutritive unless 

 they are supplied in their natural state. Pure fibrin, pure 

 gelatin, and other principles purified from the substances 

 naturally mingled with them, are incapable of supporting 

 life for more than a brief time. 



Moreover, health cannot be maintained by any number 

 of substances derived exclusively from one of the three 

 groups of alimentary principles. A mixture of nitrogenous 

 and non-nitrogenous substances, together with the inorganic 

 principles which are severally contained in them, is essen- 

 tial to the well-being, and, generally, even to the existence 

 of an animal. The truth of this is demonstrated by experi- 



