44:6 HANDBOOK OF PHYSIOLOGY. 



Gramme- 

 calories. 



120 grms. Proteid at 4, 500 per grm. = 544, 500 



90 " Fat at 9,000 per grm. = 810,000 



330 u Carbohydrate at 4000 per grm. =1,320,000 



2,694,500 



Or roughly, 2,694 kilog. calories, equivalent to 1, 144,950 metre-kilogrammes 

 of energy. This shows, although the calculation is only rough, that the diet 

 which from other reasons was considered to be correct contains the potential 

 energy to set free one million metre-kilogrammes of kinetic energy, and to 

 leave a fair margin for errors of calculation. 



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 calcu- 

 lations must be at present considered only very doubtfully approximate. 



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

 tions as the one above given 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 ex- 

 ternal 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 ex- 

 penditure in nutrition or nerve-force, it is evidently impossible to include 

 it by any method of estimation yet discovered; and all accounts of it 

 must be matters of the purest theory. These considerations may help 

 to explain the apparent discrepancy 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. 



It is of much interest to consider the way in which protoplasm acts 

 in converting food into energy plus decomposition products. It is certain 

 that the substance itself does not undergo much change in the process 

 except a slight amount of wear and tear. We may assume that it is the 



