66 HAND-BOOK OF PHYSIOLOGY. 



mechanical work; the remainder being employed in the maintenance of 

 the body's heat. The latter amount represents the heat which would be 

 required to raise 48*4lb. of water from the freezing to the boiling point; 

 or if converted into mechanical power, it would suffice to raise the body 

 of a man weighing about 150 Ib. through a vertical height of 8 J miles. 



To the foregoing amounts of expenditure must be added the quite un- 

 known quantity expended in the various manifestations of nerve-force, and 

 in the work of nutrition and growth (using these terms in their widest 

 sense). By comparing the amount of energy which should be produced 

 in the body from so much food of a given kind, with that which is actu- 

 ally manifested (as shown by the various products of combustion, in the 

 excretions) attempts have been made, indeed, to estimate, by a process of 

 exclusion, these unknown quantities; but all such calculations must be at 

 present considered only very doubtfully approximate. 



Sources of Error. Among the sources of error in any such calcu- 

 lations must be reckoned, as a chief one, the, at present, entirely unknown 

 extent to which forces external to the body (mainly heat) can be utilized 

 by the tissues. We are too apt to think that the heat and light of the sun 

 are directly correlated, as far as living beings are concerned, with the 

 chemico -vital transformations involved in the nutrition and growth of the 

 members of the vegetable world only. But animals, although compara- 

 tively independent of external heat and other forces, probably utilize 

 them, to the degree occasion offers. And although the correlative manifes- 

 tation of energy in the body, due to external heat and light, may still be 

 measured in so far as it may take the form of mechanical work; yet, in 

 so far as it takes the form of expenditure in nutrition or nerve -force, it is 

 evidently impossible to include it by any method of estimation yet dis- 

 covered; and all accounts of it must be matters of the purest theory. 

 These considerations may help to explain the apparent discrepancy be- 

 tween the amount of energy which is capable of being produced by the 

 usual daily amount of food, with that which is actually manifested daily 

 by the body; the former leaving but a small margin for anything beyond 

 the maintenance of heat, and mechanical work. 



In the foregoing sketch we have supposed that the excreta are exactly 

 replaced by the ingesta. 



NITROGENOUS EQUILIBRIUM AND FORMATION OF FAT. 



If an animal, which has undergone a starving period, be fed upon a 

 diet of lean meat, it is found that instead of the greater part of the nitro- 

 gen being stored up, as one would expect, the chief part of it appears in 

 the urine as urea, and on continuing with the diet the excreted nitrogen 

 approximates more and more closely to the ingested nitrogen u^itil at last 

 the amounts are equal in both cases. This is called nitrogenous equili- 



