(i9) 55 



engine^ utilizing only one-twentieth of the heat the 

 muscle is able to convert one-fifth of the energy of the 

 food into work. 2 ? The other four-fifths must, therefore, 

 appear as heat. Whenever a muscle contracts, then, 

 four times as much energy appears as heat as is con- 

 verted into motion. Direct experiments by Heidenhain 

 have confirmed this, by showing that an important rise 

 of temperature attends muscular contraction ; 28 a fact, 

 however, apparent to any one who has ever taken active 

 exercise. The work done by the animal body is of two 

 sorts, internal and external. The former includes the 

 action of the heart, of the respiratory muscles, and of 

 those assisting the digestive process. The latter refers 

 to the useful work the body may perform. Careful esti- 

 mates place the entire work of the body at about 800 

 foot-tons daily ; of which 450 foot-tons is internal, 350 

 foot-tons external work. And since the internal work 

 ultimately appears as heat within the body, the actual 

 loss of heat by the production of motion is the equiv- 

 alent of the 350 foot-tons which represents external 

 work. This by a simple calculation will be found to be 

 250,000 heat units, almost the precise amount by which 

 the heat yielded by the food when burned without the 

 body, exceeds that actually evolved by the organism. 

 Moreover, while the total heat given off by the body is 

 2,500,000 units, the amount of energy evolved as work 

 is equal to about 600,000 heat units ; hence the amount 

 of work done by a muscle is as above stated, one-fifth 

 of the actual energy derivable from the food. One point 

 further. The law of correlation requires that the heat set 

 free when a muscle in contracting does work, shall be 

 less than when it effects nothing j this fact, too, has been 



