43 o PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



moved through the lungs as CO s and H 3 ; but the whole 

 of the N, P, and S, together with enough C, H, and O to 

 form with the N the substance urea, are removed by 

 the kidneys. The composition of urea is CH 4 N 3 0. 



[ Urea 



and T 

 ( Salts = ( ' 



Urea 



Urine = Solution of i and ° r ' I ( Na 



Carb ' I of \ K 



[ Phos. J 



The manner in which albuminoids, both food excess 

 and waste, are divided between lungs and kidneys is 

 shown in Fig. 300, already given, but repeated here 

 slightly modified. The amyloids and fats are not elimi- 

 nated by the kid- 

 -b neys, but only the 



lungs. 



It is evident, 

 lungsV "* XIZ^- > kidneys then, that the quan- 



FiG. 300.— Diagram showing the division of titV of urea elimi- 

 albuminoids between the lungs and the kid- . . 



neys. nated is in propor- 



tion to the quantity 

 of albuminoid food, and is therefore greater in carnivores 

 than in herbivores, and also in proportion to the waste, 

 and therefore to the work done or degree of activity. 

 The estimation of the urea excreted is therefore the 

 most convenient mode of measuring waste, and is con- 

 stantly used for this purpose. 



When urea decomposes in the presence of water it is 

 wholly converted into ammonium carbonate. 



CH 4 N 2 + 2 H 2 = (H 4 N) 2 C0 3 



urea + water = am. carb. 



Comparison of Lungs and Kidneys. — The com- 

 parison between these two complementary organs is in- 

 teresting in many respects. 



