468 A MANUAL OF PHYSIOLOGY 



this, it was shown that a certain amount of work accom- 

 plished (by Pick and Wislicenus in climbing a mountain) on 

 a non-nitrogenous diet had double the heat equivalent of the 

 whole of the proteid consumed in the body, as estimated by 

 the urea excreted during, and for a given time after, the work. 

 On the assumption that all the urea corresponding to the 

 proteid broken down was eliminated during the time of this 

 experiment, a part at least of the work must have been 

 derived from the energy of non-nitrogenous material. And 

 the increase in the carbon dioxide given off, which is as con- 

 spicuous an accompaniment of muscular work as the con- 

 stancy of the urea excretion, showed that during muscular 

 exertion carbonaceous substances other than proteids that 

 is to say, fats and carbo-hydrates are oxidized in greater 

 amount than during rest. 



So the pendulum of physiological orthodoxy came full- 

 swing to the other side. Liebig and his school had taught 

 that proteids alone were consumed in functional activity ; 

 the majority of later physiologists have denied to the proteids 

 any share whatever in the energy which appears as muscular 

 contraction. The proteids, they say, ' repair the slow waste 

 of the framework of the muscular machine, replace a loose 

 rivet, a worn-out belt, as occasion may require ; the carbo- 

 hydrates and fats are the fuel which feeds the furnaces of 

 life, the material which, dead itself, is oxidized in the inter- 

 stices of the living substance, and yields the energy for its 

 work.' 



Now, it is a singular circumstance, and full of instruction 

 for the ingenuous student of science, that the facts which 

 have been supposed absolutely to disprove the older theory, 

 and absolutely to establish its modern rival, do neither the 

 one thing nor the other. The fact and it is a fact that 

 the excretion of nitrogen is but little affected by muscular 

 contraction, does not prove that none of the energy of 

 muscular work comes from proteids; the fact that, under 

 certain conditions, some of the muscular energy must 

 apparently come from non-nitrogenous materials, does not 

 prove that these are the normal source of it all. The dis- 

 tinction has again been made too absolute. The pendulum 



