Production of Heat and Work. 89 



body compares favorably with the best modern engines. In addi- 

 tion to performing external work the body must prepare its own 

 fuel, store and transport it until needed, make all repairs, and main- 

 tain a definite temperature, as Avell as direct, move, and control 

 itself. When all these functions are considered, the marvelous 

 perfection of the animal body as a machine becomes apparent. 



113. The body not a heat machine.— The animal body is not an 

 engine which converts heat into mechanical work. As Armsby^ 

 points out, the mechanical work of a steam engine is derived di- 

 rectly from the heat produced by the burning coal, but in the ani- 

 mal bod}^ the energy of the food is transformed into work in quite 

 another way. While the fuel value of a food represents the total 

 amount of energy it can liberate in the body, a varying part of this 

 total energy is always set free in the body as heat, and this heat 

 can do no external work, tho it warms the 1)ody. Only that part 

 of the food energy which is liberated in other forms than heat can 

 be utilized for the production of either internal or external work. 

 By processes still unknown the animal machine produces muscular 

 energy, heat, light, and electricity with an efficiency greater than 

 any machine made by man. AVith animals the fuel is burned at 

 low temperature. The glow worm and firefly produce light with- 

 out sensible loss of heat or other energy, and the torpedo fish and 

 electric eel generate electricity by means unknown. Scientists and 

 inventors alike are baffled by the mj^sterious and wonderful pro- 

 cesses continuously occurring in the animal body. 



As the horse is the principal animal machine for performing 

 work, this subject is appropriately continued in Chapter XVIT. 



^ Penn. Expt. Sta., Bui. 84. 



