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



in the muscle. The lactic acid on that view would be " part of the machinery 

 and not part of the fuel," to use a familiar Cambridge phrase. 



As opposed to this argument advanced by Hill we have, however, some 

 more direct evidence supplied by Paruas, whose work at Cambridge with us 

 was interrupted by the war, but has been continued in Germany (22). He 

 compared the oxygen consumed by fresh resting and by fatigued muscles 

 respectively, and thus determined the excess which was due to the process of 

 recovery. From this he calculated the total heat which would correspond, in 

 combustion, with the oxygen consumption observed. He then determined 

 the actual heat production of similar muscles during the recovery process in 

 oxygen after fatigue. This he found to be only half of that indicated by the 

 oxygen consumption, and concluded that this retained energy was stored 

 during the restoration of potential to the muscle. He suggests that the lactic 

 acid is in part burned away, the heat supplying energy for restoring what he 

 describes as the physico-chemical state of the resting muscle. Unfortunately, 

 these experiments have not yet been described in detail, and it is therefore 

 difficult to appraise their value. The technique used in the estimation of 

 the heat given out by muscles is based upon the methods developed by 

 A. V. Hill, and would appear to be satisfactory. One criticism occurs to us, 

 however. The actual amount of energy presumed to be stored as potential 

 in the fibres has relatively a very small caloric value : thus, during the 

 whole process of complete recovery from full fatigue, Parnas found that 

 only about two gramme-calories were stored per gramme of muscle. We 

 are by no means clear that any correction was made for the latent heat of 

 evaporation of water from the surface of the muscle, and this we conceive 

 might greatly affect the quantitative value of such a measurement. 



But it must be admitted that when fresh unfatigued muscles were used, 

 there was apparently a close correspondence between the heat calculated 

 from their oxygen consumption and that actually given out, which could 

 hardly have been the case if surface evaporation had been occurring. 



For the present we feel bound to conclude, upon the evidence as to heat 

 production advanced both by Hill and by Parnas, that while the lactic acid 

 produced during contraction is itself the material which is then immediately 

 oxidised with a yield of carbon dioxide, part at least of the heat of 

 combustion of lactic acid is stored in potential form in the muscle as it 

 returns to the resting state. 



We shall now endeavour to justify our belief that lactic acid itself is, as 

 a matter of fact, the material actually oxidised in muscle. 



Among those who, during recent years, have investigated, or considered 

 with expert knowledge, the intermediary processes of metabolism, not from 



