Essential Characters of Food. 29 



(aquafortis), sulphuric (oil of vitriol), lactic and 

 phosphoric, &c. 



In a chemical point of view the acids are 

 most powerful agents, and by union with various 

 substances form compounds of vital importance. 



Lastly, there are compounds of no service 

 whatever to the system. These are principally 

 indigestible bodies, such as ligneous or fibrous 

 parts of plants and foreign bodies which have 

 gained access to the digestive organs, cells, 

 earthy matter, and portions of undigested food 

 which have been present in excess. Liebig has 

 termed these the incombustible and unburned 

 parts of food. They are the refuse from the 

 digestive process, and together with effete or use- 

 less parts thrown off from the system, constitute 

 the faces or dung. 



ESSENTIAL CHARACTERS OF FOOD. 



Certain conditions are essential for the proper 

 digestion and assimilation of food. When pro- 

 perly understood, they indicate the principles of 

 an economical system of feeding animals with a 

 view towards the preservation of health and 

 vigour under continued laborious exertion, as 

 well as preventing an undue rate of mortality 

 resulting from it, and at a low rate of cost. 



These conditions are, first, that food should be 

 nutritious ; second, that it contains elements of 



