662 ANIMAL HEAT 



The Seats of Heat-Production. We have already recognized the 

 skeletal muscles as important seats of heat-production. A frog's 

 muscle, contracting under the most favourable conditions, does not 

 convert at most more than one-fourth or one-fifth of the energy it 

 expends into mechanical work; at least three-fourths or four- fifths 

 of the energy appears as heat. The muscles of mammals and of 

 man in the intact body work, upon the whole, more economically 

 than the excised frog's muscles at their maximum efficiency. Under 

 the best conditions that is, when the work is moderate and not too 



Fig. 214. Diagram showing the Heat -equivalent of various Dietaries. 

 A, proteins; B, fats; C, carbo-hydrates; D, heat -equivalent. 



rapidly done about one-third of the chemical energy expended 

 may be transformed into mechanical work, and only two-thirds into 

 heat (Zuntz). In hard work three-quarters of the energy may be 

 changed into heat; but even then the x efficiency of the muscles far 

 outstrips that of the best steam-engines, which convert only an 

 eighth of the total energy into work. 



Notwithstanding the splendid efficiency of the muscular machine, 

 the gaseous metabolism easily rises during muscular work to five 

 times, and in severe labour to nine times its resting value, although 

 persons inured to toil work more economically than amateurs. 



