144 ANIMAL CHEMISTRY LECTURE VI. 



of cold air necessary to be inspired and heated up to the tempe- 

 rature of the animal will be only half as much in the one case 

 as in the other, whereby a further economy of heat will be 

 effected. 



(151.) But in the liberation of pent-up force within the body, 

 we have to consider not only the tissue burnt, but also the oxygen 

 required to burn it. Now it is quite conceivable that in the 

 organism of birds, for instance, an economy of oxygen or breath 

 may be of more importance than an economy of tissue or food.* 

 Indeed, the marvellous adoption in predatory birds of every sup- 

 plemental contrivance for increasing ever so slightly their respi- 

 ratory surface, without, however, departing from its almost rep- 

 tilian type of structure, is strongly suggestive of these animals 

 being, so to speak, under-lunged, and of the necessity for econo- 

 mising their lung-action in every possible way. Now admitting 

 the correctness of our former conclusion, that by the transforma- 

 tion of a given amount of tissue-carbon into the carbonic oxide 

 constituent of uric acid, there is liberated say - of the actual 

 energy produceable by its oxidation into gaseous carb-anhydride, 

 it follows conversely that the transformation of two atoms of oxygen 

 into the carbonic oxide constituent of uric acid C 2 O 2 , will liberate 

 l^ times the actual energy producible by its transformation into 

 carb-anhydride CO 2 . So that while for a given amount of tissue- 

 carbon we obtain 25 per cent, less, for a given amount of oxygen 

 we obtain 50 per cent, more heat by the production of the car- 

 bonic oxide constituent of uric acid, than by the production of 

 gaseous carb-anhydride ; or an atom of oxygen is employed with 

 50 per cent, more economy in the one case than in the other. 



(152.) Moreover, the lungs, it must be remembered, act not 

 only as receivers of oxygen but also as dischargers of carbon from 



* ' Of all animals birds are most dependent upon a constant renewal of 

 the air in their lungs, and upon the purity of that with which they are sup- 

 plied. Most birds will die in air which has been but slightly charged with 

 carbonic acid, and which can be respired by mammals without immediate 

 injury.' Carpenter's 'Principles of Physiology, General and Comparative,' 

 3rd ed. p. 758. 



