INFLUENCE OF MUSCULAR WORK ON METABOLISM 



111 



When Liebig, with much greater clearness than had been attained up to 

 his time, had made out the chemical composition of foods and of the tissues 

 of the dead body, he set himself the task of determining what foodstuffs are 

 consumed in the work of the body and what significance in general the 

 different groups of organic foodstuffs have for metabolism. 



Since organisms are distinguished chemically by the fact that they con- 

 tain proteid, he assumed that the activity of the body and especially of the 

 muscles takes place at the expense of the living protoplasm, and that this 

 in turn is built up from the proteid in the food. The nonnitrogenous sub- 

 stances, he said, are used in the formation of heat in the body by direct 

 oxidation, and thus by taking possession of the oxygen they protect proteid 

 from its harmful effects. On this basis the organic foodstuffs were classified 

 as tissue forming or plastic, and heat forming or respiratory. 



The second proposition of this hypothesis can be disposed of immediately. 

 Experiment has shown definitely that decomposition of nonnitrogenous food- 

 stuffs is not inaugurated by oxygen but by the activity of the tissues. The 

 dependence of heat production upon the nervous system is evidence of this fact 

 (cf. Chapter XIV). 



If this view that bodily work takes place at the expense of the living 

 substance of the muscles, were correct, one would expect that work would 

 l)e accompanied by an increased output of nitrogen. But in the great majority 

 of the experiments made to test this point either no increase or only a very 

 slight one is found on working days. 



The following experiments by Voit on the dog will serve as an illustration: 



The same result was obtained by Pettenkoffer and Voit in their experiments 

 on man. The subject was required to turn a wheel with a crank a kind of work 

 to which he was accustomed the wheel being so loaded as to demand about the 

 same expenditure of energy as his customary work demanded. He worked nine 

 hours of the twenty-four. Fasting and resting, he gave off 12.3-12.5 g. N; 

 fasting and working, 11.7 g. On a moderate diet at rest, he eliminated 16.5- 

 17.4 g. N and at work 17.0-17.4 g. N. Here likewise there is no increase in the 

 daily excretion of nitrogen due to work. 



Fick and Wislicenus made a very important experiment on themselves. 

 They ascended the Faulhorn in Switzerland, a mountain which has an altitude 

 of 1,956 meters above the lake at its base. Seventeen hours before the ascent 

 they ate their last nitrogenous meal ; the ascent itself lasted six hours, and seven 



