INCOME AND EXPENDITURE OF BODY. 435 



converted into mechanical power,, it would suffice to raise the body of a 

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



To the foregoing amounts of expenditure must be added the quite 

 unknown 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 actually manifested (as shown by the various products of com- 

 bustion, 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 approxi- 

 mate. 



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

 tions 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 

 comparatively independent of external heat and other forces, probably 

 utilize them, to the degree occasion offers. And although the correlative 

 manifestation 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 estima- 

 tion yet discovered; and all accounts of it must be matters of the purest 

 theory. These considerations may help to explain the apparent discrep- 

 ancy between 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, 

 however, 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 nitrogen 

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

 urine as urea, and continuing with the diet the excreted nitrogen ap- 

 proximates more and more closely to the ingested nitrogen until at last 

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

 rium. There may, however, be an increase of weight which is due to 

 the putting on of fat. If this is the case it must be apparent that the 

 protoplasm of the tissues is able to form fat out of proteid material 

 and to split it up into urea and fat. If fat be given in small quantities 



