442 COMPARATIVE PHYSIOLOGY. 



Fats and Carbohydrates. — It is a matter of common observa- 

 tion and of more exact experiment that even a carnivorous ani- 

 mal thrives better on a diet of fat and lean meat than on lean 

 flesh alone. Thus, it has been found that nitrogenous equi- 

 librium was as readily established by a due mixture of fat and 

 lean as upon twice the quantity of lean flesh alone. It is plain, 

 then, that the metabolism is actually slowed by a fatty diet. 

 When an animal is given but little fat, none whatever is laid 

 up, but all the carbon of the fat can be accounted for in the 

 excreta, chiefly as carbonic anhydride. Again, the fatty por- 

 tion remaining constant, it has been found that increasing the 

 proteid leads not to a storage of the carbon of the proteid ex- 

 cess, but to an increased consumption of this element. It is 

 then possible to understand how excessive consumption of pro- 

 teids may lead, as seems to be the case, to the disappearance of 

 fat and loss of weight, so that a proteid diet increases not only 

 nitrogenous but non-nitrogenous metabolism. That carbohy- 

 drates mixed with a due proportion of the other constituents 

 of a diet do increase fat formation is well established ; though 

 there is no equally well-grounded explanation of how this, is 

 accomplished. Upon the whole, it seems most likely that fat 

 can be directly formed from carbohydrates, or, at all events, 

 that they directly give rise to fat if they are not converted 

 themselves into that substance. 



Comparative, — It is found that there are relations between 

 the food uised and the quantity of carbonic dioxide expelled 

 which are instructive. The formula following show the amount 

 of oxygen necessary to convert a starch and a fat into carbonic 

 anhydride and water : 



1. C6Hio06-l-0>»=6(COs)-l-5(HaO). 



2. C6,H, oiOe -1- 0, 60 = 57(00,) + 52(HaO). 



It will be observed that in the first case the oxygen used to 

 oxidize the starch has all reappeared as OOs, while in the sec- 

 ond only 114 parts out of 160 so reappear. As a matter of fact, 

 more of the oxygen used does in herbivora reappear as COa, 

 and less as water, while the reverse holds for the carnivora, the 

 proportion being, it is estimated, as ninety to sixty per cent. 

 This is to be explained by the character of the food in each 

 instance, for this relation no longer holds during fasting, when 

 the herbivorous animal becomes cai-nivorous in the sense that 

 it consumes its own tissues. 



