THE FACTORS WHICH MODIFY THE METABOLISM 189 



was a minimum quantity. For Fick it was 129,096 kilo- 

 grammeters; for Wislicenus, a heavier man, it was 148,656. 

 The investigators collected their urine while on the path 

 and for some hours after completing the ascent. They ate 

 only non-nitrogenous food, that the excretion might not be 

 increased merely as a result of diet. The nitrogen found 

 in the urine indicated in each case the destruction of be- 

 tween 30 and 40 grams of protein. The highest theoretic 

 contribution which such a quantity of protein could 

 have made to the work done would have been only half 

 the known total, and, of course, a smaller part of the actual 

 performance. The inference could not be escaped; some- 

 thing other than protein had been used as a source of 

 muscular energy. 



The figures obtained by Fick and Wislicenus have been 

 revised and corrected by critics in view of later discoveries, 

 but their chief significance has never been modified. At 

 the time of this celebrated experiment the respiration ap- 

 paratus at Munich had not been built. As soon as this 

 plant was in operation under the direction of Voit and 

 Pettenkofer it became possible to learn much more about 

 the effects of exercise upon metabolism. It was soon es- 

 tablished that instead of being the sole fuel employed to 

 h muscular energy, protein is not even the principal 

 The nitrogen excretion of a man on a mixed diet 

 not increase notably in a period of work as compared 

 with what it is during rest. On the other hand, his out- 

 put of carbon dioxid, like the calories, is a reliable index of 

 the degree of his activity. The contracting muscles evi- 

 dently employ non-nitrogenous material for oxidation when 

 the usual conditions of supply are maintained. 



The question which naturally follows is as to whether 

 carbohydrate or fat is the preferred fuel. This cannot be 

 discussed at length, but it seems fair to say that both may 

 be used to good purpose and with nearly equal economy. 

 The herbivorous animals may be assumed to work most of 

 the time at the expense of carbohydrate oxidized. The 

 carnivora use much more fat, though it is to be borne in 



