FOR MUSCLE WORK. 147 



It is very interesting to notice how the great truths in 

 science come to bear different interpretations in the progress of 

 knowledge. We are too apt to undervalue the discoveries of 

 our predecessors because the theories founded upon them are 

 subsequently found to be imperfect in some points. Such 

 superficial judgments of one generation are sure to be avenged 

 in the next, for at no time can any department in physiological 

 science be pronounced complete. In this question of evolution 

 of force by muscular action we have had four important prin- 

 ciples laid down at different times : 1. " The principle of Liebig, 

 that the oxidation of the albuminous matter of the muscles was 

 the sole source of the force required for mechanical work ;" 

 2. "The muscle is only the instrument through which the 

 transformation of force is brought about, but it is not the matter 

 by the metamorphosis of which the effect is produced ;" 3. " The 

 enduring power of doing work is proportional, not to the mass 

 of the muscle, but to the mass of blood circulating through it ; ;; 

 4. "Not the hundredth part of the oxidative process is per- 

 formed outside the walls of the blood-vessels." These three last 

 are laid down by J. E. Mayer, in his Mechanik der Warme 

 (see " Life and Equivalence of Force," p. 50). The last principles 

 appear conclusively to contradict the first one of Liebig, and 

 when we add to that the facts since demonstrated of the non- 

 increase of urea under severe and prolonged muscular work, 

 when the diet was non-nitrogenous, and in addition, the testi- 

 mony of Beale to the impossibility of the consumption of the 

 actual muscular structure in ordinary work, we perceive that 

 Liebig's principle, in its naked form of statement, has been pro- 

 nounced untenable with apparent justice. Nevertheless, read 

 by the light of subsequent discoveries, all of the above prin- 

 ciples contain fundamental elements of the truth, and therefore 

 constitute important stages in the progress of knowledge. 

 For we come back through Beale's theory to the principle that 

 the muscular fibre is merely a mechanical instrument for the 

 transformation of force into simple work of pulling, while the 

 evolution of the whole force takes place exclusively within what 

 is technically called the muscle, as a whole, by the consump- 

 tion of a structureless nitrogenous matter, which is constantly 



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