626 LECTURE XXVII. 



Atwater succeeded in solving a number of important problems concern- 

 ing metabolism with the help of his apparatus. If we consider metabo- 

 lism simply from the standpoint of energetics, i.e., consider the organism 

 simply as a machine, then one of the first questions to interest us is as 

 regards the utilization of the food for the performance of external work. 

 We must remember that an ordinary steam engine on an average converts 

 but 15 per cent of the energy contained in the fuel into work. The rest 

 is set free as heat. 



The table on page 625 shows the relation of the external muscular work 

 to the total amount of transformed energy, from which the efficiency of 

 the human body as a machine may be computed. 1 



The work performed in these experiments was measured by the bicycle 

 arrangement described above. In the table its heat equivalent is taken 

 into consideration. The objection might be raised that it is hardly possible 

 to distinguish sharply between a period of rest and one of work, for in the 

 latter case there is merely additional work over that required by the 

 organism for the exercise of the remaining physiological functions. It is 

 difficult, and in fact entirely impossible, to make sure that the mental and 

 nervous expenditure of energy will be exactly the same in two experi- 

 mental periods. A priori it is conceivable that there is a considerable 

 transformation of energy in these two kinds of work. To meet this objec- 

 tion Atwater 2 compared the results obtained where the person experi- 

 mented upon was resting mentally and physically as completely as possible, 

 with results obtained in a period where the person was engaged in severe 

 mental effort, and found that there was no appreciable increase in the 

 transformations of matter or of energy in the latter case. This does not 

 necessarily mean that mental activity does not correspond to a considerable 

 consumption of energy. It is entirely impossible to stop completely at 

 will all mental effort. The mental work continues, whether the person 

 experimented upon, as in Atwater's experiments was the case, is busied 

 with the results of experiments, a German treatise, or the study of physics; 

 there is no apparent difference from the results obtained in metabolism 

 when the brain is as much at rest as is possible to make it voluntarily. 

 As far as the above experiments are concerned, therefore, it makes no 

 difference to what extent brain and nervous activity affect the total 

 consumption of energy; for, as Atwater has shown, the consumption of 

 energy from these causes remains the same, whether mental work is per- 

 formed intentionally or unintentionally. A glance at the above table 

 shows that all human organisms do not work with the same degree of effi- 

 ciency. That of E. 0. was less efficient than that of J. S. F. and J. C. W. 

 At all events, however, as a machine the human organism is more efficient 



1 Atwater: loc. tit. p. 608. 



3 Atwater: U. S. Dept. Agr. Office of Exper. Stations. Bull. 44. 



