ANIMAL HEAT 501 



represented by lumberers and other out-of-door labourers 

 (who, in addition to excessive exertion, have often to fac^ 

 intense cold), the heat-production rises to 5,360,000 calories 

 The Seats of Heat-production. We have already recognised 

 the skeletal muscles as important seats of heat-production. 

 A frog's muscle, contracting under the most favourable con- 



FlG. 141. iJIAGRAM SHOWING THE HEAT EQUIVALENT OF VARIOUS 



DIETARIES. 

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



ditions, 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 rapidly done, about one-third of the chemical energy 

 expended may be transformed into mechanical work, aid 



