WORK PRODUCTION 537 



amount of material broken down in the body during work. It 

 will be convenient to consider the effects of muscular work, first 

 upon the protein katabolism and second upon the katabolism 

 of non-nitrogenous material. 



635. Early views. Since the muscles, by means of which 

 work is performed, consist largely of protein, it was not un- 

 natural for the early physiologists to suppose that the sub- 

 stance of the muscle itself was consumed and yielded the energy 

 for the work done. This was Liebig's view, although it does 

 not seem to have been based upon any actual experimental 

 results. He taught that work was performed at the expense 

 of a katabolism of protein in the muscles, causing an in- 

 creased excretion of nitrogenous by-products and an increased 

 demand for protein in the feed, while the carbohydrates and 

 fats of the feed were regarded as simply heat and fat producing 

 materials. 



636. Analogy with engine. The analogy drawn in Chapter 

 VI (274-276) between the body and an engine, however, might 

 of itself lead one to question the truth of this view. An engine 

 does not do work by burning up its own substance but by burn- 

 ing fuel material, and if it is well constructed the wear due to 

 the work imposed upon it is comparatively slight. It might 

 be reasonably expected, therefore, that the machinery of the 

 animal body would prove to be at least as perfectly constructed 

 as an artificial machine and at least equally capable of convert- 

 ing the energy of fuel material into work without destroying 

 the materials entering into its own structure. That such is 

 indeed the case under normal conditions was first shown by 

 Carl Voit, whose results have been fully confirmed by later 

 investigators. 



Voit's first investigation l was upon a dog alternately resting 

 and doing considerable work on a treadmill both when fasting and 

 upon a liberal meat diet. The results are shown in the first of the 

 two following tables, while the second contains the average results 

 of a later series of similar experiments by Pettenkofer and Voit 2 

 on a man. 



1 Untersuchungen iiber den Einfluss des Kochsalzes, des Wassers, und der 

 Muskelbewegungen auf den Stoffwechsel. 1860. Summarized by E. v. Wolff in 

 Die Ernahrung der landw. Nutzthiere, pp. 386-388. 



2 Ztschr. f. Biol., 2 (1866), 478. 



