THE PHYSIOLOGY OF ALIMENTATION. 547 



amount of heat thus lost governs the need and fixes the quantity of 

 the ration. 



Such is the theory which opposes the energetic theory and disputes 

 favor with it among physiologists. Among its strongest adherents are 

 von Noorden, Eubner, Oh. Bichet, and Lapicque. In their view the 

 generation of heat absolutely dominates the play of nutritive exchanges; 

 and it is the heat requirement which regulates the total demand in cal- 

 ories for the support of the organism. It is not at all because the 

 organism produces too much heat that it disperses it over all its surface, 

 but emphatically because that it would lose heat to a dangerous extent 

 that it is adapted to provide against the loss. 



This conception of the function of alimentation rests on two argu- 

 ments. The first is furnished by the experiments of Eubner. A dog 

 is kept a suitably long time (from two to twelve days) in a calorimeter, 

 and the quantity of heat given off is compared with the heat furnished 

 in the food. The accord between the two is in every case remarkable. 

 But would it be possible that it could be otherwise? For in a mechani- 

 cal regulator it is well known that an exact equilibrium between the 

 supply of heat and the loss must obtain for a constant temperature 

 within. The second argument is drawn from what is called the law of 

 surfaces, brought forth by Ch. Eichet. By comparing the proper rations 

 of subjects of very different weights placed in various situations, it was 

 shown that there was furnished always the same number of calories for 

 the same surface of skin — that is to say, the same cooling surface. This 

 is certainly a very interesting fact, but at the same time not necessarily 

 convincing. 



There are, on the other hand, grave objections to this view. The 

 thermal value of a nutritive principle represents only one aspect of its 

 physiological function. To be sure man and animals are able to draw 

 the same profit and the same effects from rations in which one of the 

 aliments is replaced in isodynamic proportions by two others, and the 

 same quantity of heat is thus developed. But this substitution is very 

 limited in its possibilities. Isodynamic substitution — that is, a substi- 

 tution of aliments pro rata for their thermal value, is limited on all sides 

 by exceptions. In the first place, there is a small quantity of nitro- 

 genous aliment whose place can not be supplied. Indeed, even before 

 the minimum allowance is reached the substitution seems to be perfect. 

 While substitution is perfect as between the albuminoids and carbo- 

 hydrates and the fats, it can not safely be made between these and the 

 nitrogenous substances. If the heating power of aliments was the only 

 consideration, an isodynamic substitution would not shut out alcohol, 

 glycerin, and the fatty acids from completely supporting alimentation. 

 Finally, if the thermal power of an aliment is the sole measure of its 

 physiological utility, the question arises why aliments could not be 

 wholly replaced by a dose of heat. Heating from without might, it 



