194 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



nitrogenous diet. Dogs fed exclusively on gelatin soon refuse to take 

 it, and die of hunger. Fibrin alone will also not sustain life, death 

 occurring on the fortieth to the eightieth day, while albumen, whether 

 raw or cooked, if given alone, has been found to be incapable of sustain- 

 ing life even as long as fibrin. Gluten alone has been found capable of 

 sustaining life, probably because it is not a pure nitrogenous matter and 

 alwaj's contains starch, vegetable albuminoids, and salts ; so that, there- 

 fore, the experiments made with gluten, in which nutrition seemed to be 

 tolerably well preserved, will not disprove the general statement that 

 single nutritive principles are uot capable of sustaining life. Finally, 

 again, a mixture of nitrogenous matters, such as fibrin, albumen, and 

 gelatin, although more nutritious than when any one of these is given 

 alone, is alwaj's incapable of supporting life for more than about four 

 months. 



These results show that a simple, easily - digestible substance 

 whether nitrogenous or non-nitrogenous, alone is incapable of supporting 

 life. An aliment must contain the four groups of nutritive principles 

 given above. Blood, meat, grasses, and grains are aliments, any one of 

 which, when taken alone, will sustain life. Thus, milk contains casein, 

 an albuminoid, together with albuminous bodies allied to serum-albumen, 

 which supply nitrogenous elements necessary for tissue development. It 

 contains sugar and fats for producing heat, and it contains salts in 

 amounts required for the development of all the tissues. Hay, again, 

 and grasses always contain a mixture of several kinds of plants, their 

 stems, leaves, and seeds always containing vegetable albuminous matters, 

 sugar, starch, mineral salts and fats. Observation has, however, shown 

 that the association of different alimentary substances already complex is 

 favorable to nutrition, not only by the different degrees of stimulation 

 which they exert on the different portions of the digestive tract, but by 

 the variety of nutritive matters which they render for absorption. A 

 number of apparent exceptions seem to offer themselves to the truth of 

 this statement ; thus, where we find birds feeding almost solely on a 

 single article of food, and always maintaining a high state of nutrition; 

 and yet it only requires a little reflection to show that such foods are 

 invariably themselves highly complex, and contain within them examples 

 of all the different food-stuffs. So, also, the larger ruminants will thrive 

 on a prolonged diet of any one single food; and yet here, also, the only 

 foods on which nutrition may be so preserved must be those which con- 

 tain examples of all the different food-principles. 



It is not only necessary that an aliment should contain all the differ- 

 ent food-principles, but that they should be present in considerable quan- 

 tities and in definite proportions ; otherwise nutritive equilibrium will be 

 destroyed. The essential relations between the relative proportions of 



