196 PRINCIPLES OF ANIMAL NUTRITION. 
Pettenkofer & Voit regard the slight increase in the proteid 
metabolism which they observed in most cases as a secondary effect 
of muscular exertion. They have shown, as we have seen, that 
when the cells of the body are abundantly supplied with non-nitroge- 
nous nutrients, either in the form of food or of body fat, there is a 
tendency to diminish the proteid metabolism. In work, on the con- 
trary, large amounts of non-nitrogenous material are oxidized, as 
their respiration experiments show. The supply of these nutrients 
to the cells is thus diminished, and it is to this that they attribute 
the increase in proteid metabolism. 
Results like those just given can hardly be interpreted other- 
wise than as showing that the non-nitrogenous constituents of the 
body or of the food, rather than the proteids, are the source of the 
energy expended in muscular work, but the first attempt to com- 
pare the amount of work performed with the energy available from 
the proteids metabolized was the famous experiment of Fick & 
Wislicenus * in 1866. These observers made an ascent of the 
Faulhorn and found that the amount of proteids metabolized 
during and after the ascent, as measured by the urea excreted, was 
insufficient, according to their computations, to account for more 
than one third of the energy required to raise their bodies to the 
height of the mountain, making no allowance for the work of the 
internal organs, nor for those muscular exertions which did not 
contribute directly to the work done. 
Fick & Wislicenus found no considerable increase in the uri- 
nary nitrogen in their experiment. Subsequent investigators, 
among whom may be mentioned Parkes,t Noyes,t Haughton,§ 
Meissner,|| Schenk,{{/ and Engelmann,** have reported appar- 
ently conflicting results regarding the influence of work on the 
proteid metabolism. In some cases an increase was observed, 
while in other cases no material effect was apparent. The increase 
when observed was never large except in the experiments of Engel- 
* Vrtljschr. Naturf. Gesell. Zurich , 10, 317. 
+ Phil. Mag., 4th ser., 32, 182. 
{ Amer. Jour. Med. Sci., Oct., 1867. 
§ Brit. Med. Jour., 15, 22. 
|| Virchow’s Jahresber., 1868, p. 72. 
{ Centralb. Med. Wiss., 1874, p. 377. 
** Archiv f. (Anat. u.) Physiol., 1871, p. 14. 
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