Maintenance of Muscular Effort 167 



activity whieh no other nutrients have ? The belief 

 prevailed at one time that muscular contraction caused 

 a wasting of the muscle substance which must be re- 

 placed by the proteid compounds of the food; in other 

 words, protein alone was believed to sustain the work of 

 the animal body, both internal and external. It would 

 follow from this that the more work is done the more 

 protein is needed. This view is no longer held. The 

 more exact methods of modern research have revealed 

 the fact that an increase of muscular effort, even up to a 

 severe point, increases but little, if any, the nitrogen 

 compounds of the urine, these being the measure of the 

 protein that is destroyed. There has come to light a 

 corresponding fact that the consumption of fuel in the 

 body other than proteids increases proportionately with 

 the increase of work. This means that as animals are 

 ordinarily fed mechanical work is largely sustained 

 through the combustion of carbohydrates and fats, and 

 that while for reasons we do not yet wholly understand 

 a fairly generous amount of protein seems to promote 

 the well-being of a draft animal, the non -nitrogenous 

 nutrients mostly supply the extra energy demanded for 

 the labor. 



Heat relations. — The question is very naturally 

 asked, As no energy is lost, into what is the energy of 

 muscular contraction converted, as, for instance, that 

 required for walking, the beating of the heart and the 

 work of the intestines ? It is concluded bj' physiologists 

 that muscular energy used by the animal is partly trans- 

 formed into external motion and partly into heat, and 

 this certainly is consistent with facts as observed. Vio- 



