684 



PHYSIOLOGY 



be done in a calorimeter and measuring the total output of heat by the 

 individual at rest and during work, or by calculating from the respiratory 

 exchanges the amount of heat which must be set free by the oxidation of 

 the constituents of the body consumed in the performance of muscular 

 work. In the following Table are given the results of an experiment by 

 Atwater in which the total energy output of a man was reckoned in the 

 form of heat by means of the calorimeter. The work done by the man 

 on a stationary bicycle with a brake was also accurately determined, and 

 is given in the Table in terms of its heat equivalent, viz. as Calories. 



ENERGY PER DAY 



In the above experiments, the mean expenditure of energy on the work 

 days was 5120, and on the rest days (with food) 2397 Calories, a difference 

 of 2723 Calories. This represents the excess metabolism over resting require- 

 ments, which must take place in order that the body may perform work equal 



to 546 Calories. The fraction 



total energy evolved minus resting metabolism 



is spoken of as the ' mechanical , efficiency ' of the body. In the above 

 case it equals one-fifth, and the .body is said to have a 20 per cent., 

 efficiency. Of course a certain proportion of this excess of energy over 

 work done is accounted for by the increase in the work which must be 

 performed by the respiratory muscles and heart in the state of greater 

 activity which is imposed upon them by the external work, and is necessary 

 for the proper provision of the active muscles with increased food-supply 

 and oxygen. Even if we neglect these factors altogether, the efficiency 

 of the body as a machine corresponds to between 16 and 25 per cent., an 

 efficiency which exceeds that of the best of our steam-engines and is only 

 (Mpialled by certain internal-combustion engines. 



In some cases the mechanical efficiency may attain even a higher figure, 

 so that as much as 30 per cent, of the increased metabolism accompanying 

 muscular work may be transformed into mechanical energy. Can we 

 employ these results to throw light on the nature of the constituents of 

 the body which undergo oxidation to furnish the energy of muscular work? 



