PRODUCTION OF HEAT AND WORK. 119 



Production of Heat. In the case of an animal at 

 rest, not gaining in weight, the final results of the 

 digested food will almost wholly appear as heat and 

 excrementitious matter. 



The smaller the animal the larger is the production 

 of heat per unit of weight. Thus a full-grown dog, 

 weighing 6 Ibs., was found to produce twice as much 

 heat per unit of weight as one weighing 40 Ibs. This is 

 due to the fact, that small bodies have, in proportion to 

 their weight, a much greater surface than large bodies, 

 and consequently suffer more by cooling. Small animals 

 thus stand in special need of a liberal diet. 



Production of Work. The work performed by an 

 animal is partly internal and partly external. The 

 internal work consists in the muscular movements which 

 produce circulation, respiration, and other vital pro- 

 cesses ; such work is carried on even when the animal 

 is at rest. In man the whole of the blood is pumped 

 through the heart every half minute. The daily work 

 performed by the heart of an average man has been 

 calculated as equal to 150 200 foot-tons ; that is to 

 say, the power exerted by the heart would raise 1 ton 

 to the height of 150 200 feet* The work performed 

 by other organs, and by the muscles when merely 

 maintaining the body in an erect position, must be very 

 considerable, but has not yet been satisfactorily meas- 

 ured. Nearly the whole of the internal work is finally 

 resolved into heat. 



As external work we may take as an example a walk 

 of 20 miles on level ground ; this to a man of 11 stone 

 weight (154 Ibs.) will represent an exertion equivalent 



