420 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



burn into C0 2 and H 2 without residue, and are elimi- 

 nated by the lungs alone. They do not form tissue. 

 They are used for fuel only, {b) Albuminoid excess— 

 i e albuminoid over and above what is necessary for 

 tissue building, both repair and growth, (c) Waste tissue. 

 These last two— i. e., albuminoid food excess and waste 

 —consist of C, H, O, and X, and often a small quantity 

 of sulphur and phosphorus. They are not wholly com- 

 bustible into CO s and 

 H 2 0, and wholly elim- 

 inable by the lungs. In 

 burning they leave an 

 incombustible residue 



CCtHO* XZ^->u,re,a 



Fig. 293. — Diagram showing the splitting 



of albuminoids in a combustible and to be eliminated by the 

 incombustible portion. ... r^ u „„ 



kidneys. They are, as 

 it were, split into two parts, a combustible and an incom- 

 bustible. The combustible, consisting of the larger por- 

 tion of the C, H, and 0,is eliminated by the lungs, but a 

 portion of the C, H, and O, together with the whole of 

 the N, is eliminated as urea by the kidneys. This is 

 diagrammatically represented by the formula (Fig. 293), 

 in which the line a b represents the line of splitting. 

 Therefore these two organs, the lungs and the kidneys, 

 are complementary to one another. They divide the 

 albuminoid food excess and the waste between them. 

 But the amyloids and fats are disposed of only by the 

 lungs. 



We have seen that the three kinds of fuel used are (1) 

 amyloids and fats ; (2) albuminoid excess; and (3) waste. 

 Now, since in the mature body the repair just balances 

 the waste, it is evident the fuel burned is exactly equiva- 

 lent to the whole of the food. 



4. But the question occurs: (1) How is force created 

 by combustion ? and especially (2) How can force enough 

 be generated not only to do the work of repair and main- 



