MUSCLE. 29 







to deflection of the thread of the galvanometer ; the amount of 

 deflection produced by a unit of heat is previously determined. 



Experiments made with this apparatus show that the production 

 of heat in muscle occurs both during and for some time after its 

 contraction. 



(1) When a muscle contracts, a certain amount of potential energy 

 is liberated as free energy, which can be used in the carrying out of 

 work, or can be evolved as heat, or may be manifested partly as work 

 and partly as heat. The amount of the energy set free and available. 

 for this purpose depends upon the initial length of the muscle fibre. 



If the muscle is stretched by a spring or weight, the energy set 

 free and the contractile stress, that is, the force with which the fibres 

 tend to contract, are greater than when it is not stretched. The 

 proportion of the energy thus set free, which appears as work or as 

 heat respectively, depends upon the mechanical conditions under which 

 the muscle is placed. If the muscle is allowed to lift a weight, it does 

 work which is measured by the product of the weight raised and the 

 height through which it is lifted ; and if the mechanical conditions are 

 favourable, the greater part of the energy set free during the contrac- 

 tion may appear as work. If the muscle is not allowed to contract and 

 to do work, the whole of the energy set free during the contraction 

 appears as heat. 



(2) After the contraction is over, a further evolution of heat takes 

 place, which is due partly to the oxidation of lactic acid, and partly 

 also to the rebuilding of the precursor of lactic acid, the energy needed 

 for this process being supplied by the oxidation of other substances in 

 the muscle with the liberation of heat and of carbonic acid. 



The efficiency of muscular contraction is the proportion of the total 

 energy set free during and just, after the contraction, which appears as 

 work. The mechanical conditions under which muscular contraction 

 takes place in the body are usually such that the efficiency is com- 

 paratively constant ; in man, from 20 to 28 per cent, of the total energy 

 set free during muscular contraction appears as work. 



Fatigue and Rigor Mortis. When a muscle passes into rigor mortis 

 it shortens slightly, becomes rigid and opaque, and loses its elasticity 

 and extensibility. During its passage into rigor the muscle becomes 

 acid, the acid formed being lactic acid, and heat and carbonic acid are 

 evolved. 



These changes are all due to the breaking down of the carbohydrate 

 precursor of lactic acid. In this process heat is evolved, and the lactic 

 acid reacts with the sodium carbonate in muscle, setting free carbonic 

 acid, which is given off. When the amount of lactic" acid formed in the 



