54 (t8) 



traction of muscle, was used to show the conversion of 

 mass-force into molecular force. No one in this room 

 believes, I presume, that the result would have been at 

 all different, had the motion been supplied by a steam- 

 engine or a water-wheel. Again, food, as we have seen, 

 is of value for the potential energy it contains, which 

 may become actual in the body. Liebig, in 1842, as- 

 serted that for the production of muscular force, the 

 food must first be converted into muscular tissue, 2 3 a 

 view until recently accepted by physiologists.^ It has 

 been conclusively shown, however, within a few yeare, 

 that muscular force cannot come from the oxidation of 

 its own substance, since the products of this metamor 

 phosis are not increased in amount by muscular exer- 

 tion.^ Indeed, reasoning from the whole amount of such 

 products excreted, the oxidation of the amount of mus- 

 cle which they represent would furnish scarcely one- 

 fifth of the mechanical force of the body. But while 

 the products of tissue- oxidation do not increase with 

 the increase of muscular exertion, the amount of car- 

 bonic gas exhaled by the lungs is increased in the exact 

 ratio of the work done. 26 No doubt can be entertained, 

 therefore, that the actual energy of the muscle is simply 

 the converted potential energy of the carbon of the food. 

 A muscle, therefore, like a steam-engine, is a machine 

 for converting the potential energy of carbon into motion. 

 But unlike a steam-engine, the muscle accomplishes this 

 conversion directly, the energy not passing through the 

 intermediate stage of heat. For this reason, the mus- 

 cle is the most economical producer of mechanical force 

 known. While no machine whatever can transform all 

 of the energy into motion the most economical steam 



