CONTRACTILE TISSUES 



445 



contraction and concerned with the restoration of the system to its initial state, 

 with the replacement of the lactic acid. The oxidation process, then, has only 

 an indirect relationship to the actual contractile process, although it is, of course, 

 an essential one, since it provides energy for the subsequent work to be done 

 by the muscle in contracting. Verzar (1912) showed that the consumption of 

 oxygen by the gastrocnemius muscle of the cat was increased for some minutes 

 after the end of a tetanus of about half a minute; Hill (1911, 1) showed that 

 production of heat in the frog's muscle continued for a considerable time after 

 the end of contraction and also (1913, 1, p. 43) that, dunng this after-period, 

 the heat production associated with the recovery process is about equal to that 

 obtained in the contractile process when the tension is allowed to become 

 degraded to heat. Peters (1913, p. 264) showed that the heat evolved when 

 a muscle is stimulated to fatigue in absence of oxygen is about 0'9 calorie per 

 gram of muscle. Therefore, we may reckon, from Hill's result, that the heat 



Gms 



40 



30 



20 



10 



Hours 



10 



20 



30 



40 



50 



FIG. 137. REPLACEMENT OF LACTIC ACID IN MUSCLE AFTER CONTRACTION. At the 

 beginning there are two estimations of maximum lactic acid, produced by heat rigor, 

 in control muscles. At the end there are two similar estimations which have the 

 same value, although the muscles had gone through nine periods of severe stimulation, 

 alternating with rest in oxygen. 



The enclosed areas represent time periods of stimulation. 



x , Loss of excitability. 



Temperature, 15. 



The continuous part of the line shows the course of acid loss in oxygen as actually determined by 

 estimation. The dotted line shows the presumed course of acid loss and gain during the other periods 

 of rest and stimulation respectively. 



(Fletcher and Hopkins, 1907, p. 293.) 



evolved in the oxidative process of recovery is another 0'9 calorie, being equal 

 to that produced in the contractile process. We require further to know the 

 amount of lactic acid which disappears in the recovery process, which we can 

 obtain from the work of Fletcher and Hopkins (1907). The maximum yield in 

 heat rigor is 0*003 to 0'004 g. per gram of muscle and, when stimulated to fatigue 

 about half of this (p. 280). We have now the fact that the disappearance of 1 g. 

 of lactic acid, in the recovery process, is associated with the production of 450 

 calories (see the paper by A. V. Hill, 1914, 2). But 1 g. of lactic acid on 

 oxidation gives 3,700 calories, or eight times as much as that actually obtained 

 in the muscle process, a difference far too large to be experimental error. We 

 must conclude that if lactic acid is oxidised at all, only a small part of it 

 disappears in this way, and the results of Fletcher and Hopkins show that 

 no detectable part is oxidised in any case. 



For example, in their experiment of p. 293, about 0'5. g. of lactic acid had been produced 

 and had disappeared again, but, at the end, the total amount in rigor was rather higher than 

 that of the companion set, 0'540 against 0-500. If only one-eighth had been oxidised, its 

 disappearance would have been detected. 



