RATIO OF CARBON TO NITROGEN IN UREA. 145 



the body, and, in this particular, we find the lungs of birds mate- 

 rially aided by the ejection from their kidneys of urate of am- 

 monia instead of urea. In urea CN 2 H 4 0, the ratio of nitrogen 

 to carbon is as 2 to i ; whereas in urate of ammonia C 5 N 5 H 7 3 , 

 it is as i to i , so that for an equal elimination of nitrogen, the 

 quantity of carbon discharged from the kidneys of birds in the 

 form of nrate of ammonia, is twice as great as that discharged 

 from the kidneys of mammals in the form of urea. Now, we have 

 seen that the proportion of the carbon of nitrogenised tissue 

 CsNjjH^O^, excreted by the kidneys of carnivorous mammals is -| 

 of the whole, and accordingly that excreted by the kidneys of car- 

 nivorous birds in the form of urate of ammonia will be - of the 

 whole. In other words, by the production of urate of ammonia 

 instead of urea, the lungs of birds are required to discharge only 

 - instead of | of the waste carbon resulting from the metamor- 

 phosis of nitrogenous tissue, although, as I have already observed, 

 a larger amount of nitrogenous tissue has to be transformed in 

 order to liberate the same amount of energy. On this view the 

 comparatively large kidneys of birds and insects will have refe- 

 rence not only to the absolute amount of tissue metamorphosed, 

 but also to the relative increase in the proportion of carbon ex- 

 creted by their kidneys to that excreted by their lungs. 



(153.) To sum up the foregoing considerations, we perceive 

 that the amount of force lost by the metamorphosis of a given 

 quantity of tissue, or indirectly of food, into the carbonic oxide 

 constituent of uric acid, instead of into carb-anhydride, is less 

 than might at first be supposed that the amount of force gained 

 by the conversion of a given quantity of oxygen into the carbonic 

 oxide constituent of uric acid, instead of into carb-anhydride, is 

 very large and lastly, that by the production of the carbonic 

 oxide constituent of uric acid, instead of carb-anhydride, the kid- 

 neys of birds are enabled to act vicariously for the lungs as ex- 

 cernents of a portion of that carbon which in mammals is habit- 

 ually discharged by the lungs, 



How far this production and excretion of urate of ammonia, 

 instead of urea and carb-anhydride, may depend upon an inade- 



L 



