542 NUTRITION OF FARM ANIMALS 



can be determined, have led to the extensive use of this method 

 for investigations upon work production. 



640. Nature of non-nitrogenous material katabolized. 

 Since under normal conditions muscular exertion does not in- 

 crease the protein katabolism, it follows that the substances 

 oxidized for the performance of work must be substantially 

 either carbohydrates or fats. If the former, each volume of 

 carbon dioxid given off will correspond to an equal volume of 

 oxygen taken up ; that is, the respiratory quotient (296) will 

 be i.o. On the 'other hand, if the material oxidized consists 

 solely of fat, the respiratory quotient will be approximately 

 0.7, while if both are being consumed, it will have an intermediate 

 value. Moreover, it is comparatively simple to calculate from 

 the respiratory quotient the proportions in which the two are 

 being katabolized. Investigations of this sort show that the 

 proportions of fat and carbohydrates katabolized for the per- 

 formance of work may vary within wide limits, both groups 

 being readily available as sources of energy. 



Sources of energy for muscular work 



641. Proteins vs. non-nitrogenous matter. Liebig's as- 

 sumption (635) of an increase of the protein katabolism in 

 muscular contraction implied that the proteins were the source 

 of the energy manifested, and this view prevailed for many years. 

 When Voit, in 1860, showed (636) that muscular exertion is not 

 necessarily accompanied by any material increase in the protein 

 katabolism, the inference seemed unavoidable that non-nitrog- 

 enous materials were the main sources of muscular energy. 

 This conclusion, however, was too radical to be at once ac- 

 cepted in opposition to Liebig's authority and numerous in- 

 genious, but not always convincing, hypotheses were advanced to 

 explain the observed phenomena on the assumption that the 

 proteins were, nevertheless, the source of the energy expended. 



642. Fick and Wislicenus' experiment. The first attempt, 

 however, at a quantitative comparison of the work performed 

 with the energy available from the protein katabolized during 

 its performance was the famous experiment of Fick and Wis- 

 licenus 1 in 1866. These observers made an ascent of the Faul- 



1 Vrtljschr. Naturf. Gesell. Zurich, 10, 317. 



