594 PURPOSES SERVED BY RESPIRATION. 



There appears, therefore, to he a beauliful adaptation to the wants and 

 convenience of animals in the large proportion of starch, gum, and sugar, 

 which the more abundant varieties of vegetable food contains. In obtaining 

 carbon from these, the least possible labour, so to speak, is imposed upon 

 the digestive organs of the herbivorous races. The starch and sugar 

 abound because much carbon is required, while fatty matter or oil is 

 present in smaller quantity, because comparatively little of this is neces- 

 sary to the performance of the usual healthy functions of the animal 

 body. And it is another adaptfition of the living body to the circum- 

 stances in which it may be pla ,'3d, that when starch or sugar cannot be 

 obtained, the oil of the food is consumed for the supply of carbon to the 

 lungs — and failing this also, the gluten and albumen of the vegetable food 

 or the muscular fibre of the animal food, or even of the living animal it- 

 self. 



3°. Purposes served by res}dratioii. — But for what purpose essential to 

 life do animals respire ? If the starch and sugar be so necessary to feed 

 the respiration — the breathing itself must be of vital importance to the 

 living animal. 



Some doubts still exist upon this point. It is generally believed, 

 however, that carbon is consumed or given off from the lungs for the pur- 

 pose of sustaining the heat of the living body. When starch, or sugar, 

 or gum, are burned in the open air, they are changed into carbonic acid 

 and water, and at the same time produce much heat. It is supposed that 

 in the body the same change — the conversion of starch and sugar into 

 carbonic acid and water — taking place, heat must in like manner be pro- 

 duced. A. slow combustion, in short, is su[)posed to be going on in the 

 interior of the animal — the heal of the body being greater, in proportion 

 to the quantity of carbonic acid given off from the lungs. In favour of 

 this view many strong reasons have been advanced, but there are also 

 objections against it of considerable weight, which cannot as yet be satis- 

 factorily removed. 



Were we to adopt this opinion in regard to tlie main purpose served by 

 respiration as the true one, it would afford a very distinct reason for the 

 large amount of starch existing in all our cultivated crops. Respiration, 

 according to this view, is necessary to supply heat to the animal, and 

 this respiration is most simy)ly and easily fed by the starch contained in 

 the vegetable food. The life and labours of the plant again minister to 

 the life and labours of the animal. 



§ 4. Of the origin and the purposes served by the fat of animals. 



1°. The immediate origin or source of the fat of animals depends upon 

 the kind of food with which the animal is fed. Carnivorous animals 

 obtain or extract it ready formed from the flesh they eat — herbivorous 

 animals from the vegetable food on which they live. 



It has only been lately shown that the corn, hay, roots, and herbage, 

 on which cattle are fed, contain a sufficient quantity of oily matter ready 

 formed to supply all the fat which accumulates in their bodies — or which, 

 by the milk cow, is yielded in the form of butter. Before t lie different 

 kinds of food had been analyzed, with the view of determining the quan- 

 tity of oil and fat they severally contain, it was supposed that the fat of 

 animals was derivefi almost solcty from the starch and sugar or gum, of 



