CHANGES m THE BLOOD. 91 



siTgar, oil, and the mineral salts, more particularly the phosphate 

 of lime and the chloride of sodium, with iron, potash, &c. ; and 

 it has been proved that all nutritive food must contain the 

 three principles, and that the absence of any one of them induces 

 starvation and death. It is also proved that the chemical con- 

 stitution of plants and animals is nearly identical, and hence 

 food derived from plants contains the substances of which the 

 animal body is made, and by which it is nourished. Some 

 animals are carnivorous, and must be maintained on flesh. In 

 others life is supported by plants and grain. The digestive 

 organs differ somewhat in these; but the digestion — that is 

 to say, the ultimate end of the digestive process — is the same in 

 all, namely, the formation of a whitish emulsion, the chyle, 

 which is absorbed from the intestinal canal, and conveyed into 

 the general circulation, there to be transmitted to all parts of 

 the body, giving to each materials which it may at the time 

 stand in need of for its development, growth, and vitality. In 

 all animals water is also necessary, not only as a diluent, but as 

 forming a component part of the blood and tissues. 



It must be understood that mere nitrogenous, non-nitrogenous, 

 and saline materials of food will not serve the purposes of 

 healthy nutrition; they must be blended and united to form 

 the kind of food natural to each class of animal, and capable 

 of being finally converted into albumen, fat, and salts, as all 

 these exist in every tissue ; the fibrous and muscular organs 

 containing much albumen, the glands and areolar tissue fat, 

 and the bones mineral matters. 



The quantity of food required is greatly influenced by the 

 weather. If cold and condensed, the air contains more oxy- 

 gen, which, rapidly uniting with the tissues for the purpose 

 of maintaining the normal standard of animal heat, the body 

 demands more nourishment to prevent emaciation. Again, work 

 or severe bodily exercise causes waste of tissue, and at the same 

 time, if not too severe, stimulates the appetite, and improves the 

 powers of digestion. The demand for more food, the increased 

 appetite, and the exalted digestive powers being necessary to the 

 wellbeing of the animal, as compensating agents, by which 

 more food is partaken of and digested, in order to make up the 

 waste of tissue induced by the bodily exercise, a more or less 

 even balance is thus kept up between the secondary digestion, 



