MUSCULAR CONTRACTION ii 



utilised in part for the resynthesis of the remaining lactic 

 acid. About one-fifth of the lactic acid removed appears to 

 be oxidised. The sequence of phenomena is represented 

 diagrammatically in Fig. 2. 



{d) The Nature of the Mechanism. — In considering the 

 heat-production of muscle in relation to the chemical events 

 of the contraction cycle, the muscle has been assumed to 

 respond under conditions in which no mechanical work is 

 done, so that all the energy set free at excitation is measured in 

 heat units. It has been shown, however, that we can determine 

 how much of this energy is available for the performance of 

 mechanical work in appropriate circumstances. The potential 

 energy available for the performance of mechanical work in a 

 single twitch is T//6. When the value of H for initial heat- 

 production (heat-production without recovery) in isometric 

 contraction is reduced to the same units, the ratio T//6H 

 expresses the absolute mechanical efficiency of muscle ; and 

 its value is found to approximate to unity (as high as 0-91 in a 

 series of A. V. Hill's experiments) in a suitable muscle such as 

 the frog's sartorius. This implies that the whole of the energy 

 liberated at excitation is free energy, i.e. capable, unlike heat, 

 of being transformed entirely into mechanical work. 



Heat, as implied in the Second Law of Thermodynamics, 

 cannot be transferred into mechanical work without waste. 

 No heat engine can be more efficient than a reversible engine 

 working between the same temperature-limits, i.e. with a 

 maximal efficiency of T— T'/T on the gas thermometer scale. 

 Thus, in order that a frog's sartorius may have an efficiency of 

 0-25 (which is actually realisable), it would be necessary, on 

 the assumption that the energy was derived from the heat 

 produced, for the muscle to be raised to a temperature above 

 the boiling-point of water. The muscular machine is not 

 therefore a heat engine ; and the fact that all the energy set 

 free at excitation is available for doing mechanical work signifies 

 that the exciting substance must he liberated at the seat of tension ; 

 if it were not so, a large part of the energy liberated would be 

 irreversibly degraded into heat. 



It is not difficult to construct a mechanical model to show 



