THE LAWS OF ENERGETIC EXPENDITURE 183 



the tools on which they act, and because, it is compounded of 

 uncertain elements, such as the skill of the workman and the 

 perfection of the tools. It follows that the estimation of the 

 energetic expenditure, either according to the consumption of 

 oxygen, or according to the living ration, is the most reliable 

 guide to the magnitude of the muscular work and the degree of 

 fatigue at any moment, and for the relative comparison of opera- 

 tions that are not directly comparable. The appearance of 

 physiological troubles will mark the limits of fatigue. This 

 indication will be developed further in regard to the subject of 

 industrial work. 



126. The Running of the Human Machine. The Law of Repose. 



The evaluation of the expenditure of energy the accurate 

 criterion of the amount of work performed by the muscles must 

 be made when the subject is in a normal state, as to his mechani- 

 cal action, his respiration, and his gaseous exchanges. It is 

 difficult to obtain a perfectly constant speed in ordinary machines. 

 Man readily attains it, owing to the co-ordination of sensations 

 and movements, and to what is also called the muscular sense or 

 the sense of resistance, in short, to the nervous centres, f 1 ) The 

 initial period of getting into action is all the quicker, the longer 

 experience the subject has had of the work and the smaller his 

 personal equation ( 83). Normal working continues as long as 

 fatigue has not decreased the nervous excitability. In extreme 

 cases this fatigue is felt in the nervous centres themselves and 

 disturbs the co-ordination. 



Certain features in the operation of the human machine must 

 be pointed out ; they reveal themselves in the energetic expendi- 

 ture. This expenditure, after the commencement of an action, 

 accelerates rapidly and the oxygen consumed in the first two 

 minutes so far exceeds the quantity of carbonic acid gas eliminated 



CO 2 



that the respiratory quotient -^ undergoes a fall of about 15%. 



Then the respiration rapidly becomes regular. 



In recommencing an action several times, after intervals of re- 

 pose of a few minutes, a less noticeable lowering of the respiratory 

 quotient is found, which would seem to indicate an adaptation of 

 the subject which lessens the expenditure. ( 2 ) 



The normal state being once attained, the expenditure in- 

 creases little by little ; muscular groups, up till then inactive, 

 intervene in the work ; the chemical reactions which furnish the 

 energy are wider and more rapid. At the end of half an hour, 



( x ) S. A. Pari and Farini (Atti del Real Istitute Veneto, vol. xiv., p. 929, 

 1904-5). 



( a ) Jules Amar (Comptes Rendus Sciences, 1910, vol. cli., p. 680). 



