78 CONTRACTILE ELEMENTS. 



The material^ of this active combustion are principally 

 hydrocarbons, that is, the fatty and amyloid substances pro- 

 duced in the blood ; in other words, the aliments called 

 respiratory, for the muscle oxidizes scarcely any nitrogenous 

 substances, and muscular labor causes scarcely any increase 

 in the excretion of the urea. 1 



1 The fact that the muscle, when at work, consumes principally 

 hydrocarbon aliments, and not albuminoid substances, is quite a 

 recent acquisition to science, and is a part of the knowledge lately 

 obtained as to the mechanical equivalent of heat. 



Liebig had divided all aliments into respiratory and plastic. The 

 former, by their combustion, produced animal heat ; these were 

 the fatty substances and sugars, hydrocarbons, in short: the latter, 

 represented by the albuminoids, were intended to repair the tis- 

 sues, especially the muscles. As to muscular labor, it was pro- 

 duced by the muscle at the cost of its own substance, the albuminoid 

 aliments thus alone supplying the material for it. 



The new ideas as to mechanical labor, and its relations to 

 heat, derived from the researches of Rumford, of Tyndall, of 

 Joule (Manchester), of Mayer (Bonn), and Hirn (Logelbach), 

 showed that heat and mechanical labor are the same thing, or, at 

 least, are two equivalent forces;* that one is transformed into the 

 other, according to the law of the equivalence and constancy of forces., 

 and that, for instance, a calorie may be made to produce 425 kilo- 

 grammetres; that is to say, that the great heat which raises one 

 kilogramme of water one degree can also, under another form 

 (labor), raise a weight of one kilogramme to the height of 425 

 metres: thus the number 425 expresses the mechanical equivalent of 

 heat. 



Now the muscle is also a machine ; it transforms heat into me- 

 chanical labor (see the text, some lines farther on), only it is a 

 more perfect machine, than any manufactured one, a machine 

 which, with a much smaller weight, transforms into labor a far 

 larger part of the heat produced (one-fifth in place of one-tenth, 

 as given by the best steam engines). 



If, then, we consider muscular labor as transformed heat, its 

 source must lie in the combustions which produce heat, and the 

 muscle must be looked upon, not as an apparatus which consumes 

 its own substance, but as one which serves as a place of combustion 

 for the materials which produce heat or labor. This was the hypoth- 

 esis put forth by Mayer, in 1845, when, relying on the principle 

 of the constancy of forces, he first looked upon heat and muscular 

 labor as manifestations of living forces, and considered them as 

 emauating from one and the same origin, combustion. 



From that time the division, as made by Liebig, of aliments 



* See Paul Bert: Article "Chaleur," in the "Nouveau Dictionnaire de Me"de- 

 tine et de Chirurgie Pratiques," Vol. VI. 



