460 Dr. W. M. Fletcher and Prof. F. G. Hopkins. 



recover in oxygen, again stimulated, and again submitted to oxygen, these 

 alternate processes being many times repeated. It is clear that, since 

 lactic acid is produced during each period of stimulation, and removed 

 during each period of recovery in oxygen, heavy drafts must be made 

 upon the precursor of the acid in any experiment such as that described. 

 Nevertheless, a set of muscles, after having undergone such treatment, give, 

 when thrown into heat rigor, exactly the same maximum yield of lactic 

 acid as a set of perfectly fresh muscles (fig. 8). 



Hours 10 20 ' 30 40 50 



Fig. 8. — The relation of the heat-rigor lactic acid " maximum " to the survival history of 

 muscle. Four estimations of lactic acid due to heat rigor are shown, two at the 

 beginning, in the case of resting muscles, two at the 53rd hour, in the case of 

 inexcitable muscles, which had gone through nine periods of severe stimulation 

 alternated with periods of rest in an oxygen atmosphere. The enclosed areas 

 represent time periods (drawn proportionate to abscissae) of stimulation by strong- 

 interrupted shocks, x loss of excitabilhVy. Temperature 15° C. Continuous line 

 shows course of acid loss as actually determined by estimation. Dotted line shows 

 the presumed course of acid loss and gain during other alternate periods. (From the 

 1 Journal of Physiology,' vol. 35, p. 293 (1907).) 



In discussing these results, we suggested as a possible explanation (though 

 we discussed alternative possibilities) that lactic acid is not oxidised during 

 the recovery of the muscle, but is rebuilt into the complex from which it was 

 derived, at the expense of energy derived from the oxidation of something 

 else. The formation of this unstable complex would then be the basis for 

 that restoration of potential which we have just been considering. 



This view has been widely adopted, but we ourselves are now disposed to 

 doubt it. It was shown at Frankfort by Kondo,* in Embden's laboratory, 

 that the formation of lactic acid in expressed muscle-juice is clue to a 

 * ' Biochemische Zeitschrift,' vol. 45, p. 63 (1912). 



