DOMESTICATED ANIMALS. 83 



move more freely in the air, or compelled to draw 

 heavy burdens, the fat again disappears. 



It is evident, therefore, that the formation of fat 

 in the animal body is the result of a want of due 

 proportion between the food taken into the stomach 

 and the oxygen absorbed by the lungs and the skin. 



A pig, when fed with highly nitrogenised food, 

 becomes full of flesh ; when fed with potatoes 

 (starch) it acquires little flesh, but a thick layer of 

 fat. The milk of a cow, when stall-fed, is very rich 

 in butter, but in the meadow is found to contain 

 more caseine, and in the same proportion less butter 

 and sugar of milk. In the human female, beer and 

 farinaceous diet increase the proportion of butter 

 in the milk ; an animal diet yields less milk, but it 

 is richer in caseine. 



If we reflect, that in the entire class of carnivora, 

 the food of which contains no substance devoid of 

 nitrogen except fat, the production of fat in the body 

 is utterly insignificant ; that even in these animals, 

 as in dogs and cats, it increases as soon as they live 

 on a mixed diet; and that we can increase the forma- 

 tion of fat in other domestic animals at pleasure, but 

 only by means of food containing no nitrogen ; we 

 can hardly entertain a doubt that such food, in its 

 various forms of starch, sugar, &c., is closely con- 

 nected with the production of fat. 



In the natural course of scientific research, we 

 draw conclusions from the food in regard to the 

 tissues or substances formed from it ; from the ni- 



