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A TEXTBOOK OF PHYSIOLOGY 



is to say, 50 per cent, of the energy of the foodstuffs katabolized may 

 appear as work, an efficiency much greater than that of a steam 

 engine (10 per cent.), and greater even than the best petrol engine 

 (30 per cent.).* In the muscle machine, the " free energy " which is to 

 become mechanical or thermal energy is stored in certain unstable 

 -chemical compounds, one of which is possibly the lactic acid precursor. 

 During the preliminary stage of the contractile process, certain 

 molecules are liberated in the muscle under the influence of oxygen 



6O 



FIG. 295. GALVANOMETER DEFLECTION SHOWING FALL OF TEMPERATURE OF MUSCLE 

 AFTER EXCITATION IN NITROGEN, IN OXYGEN, AND WARMED WHEN DEAD AS 

 CONTROL. (A. V. Hill.) 



The curve for living muscle in nitrogen nearly coincides with control curve, but the 

 curve for living muscle in oxygen after nitrogen is very considerably displaced 

 to the right, showing continued heat production during relaxation of the muscle. 



and with the production of heat for example, lactic acid from its 

 unstable precursor. These produce " tension energy," occasion 

 the contraction, and are then, under the further action of oxygen 

 during the relaxation, removed or replaced in the muscle substance 

 complex with the evolution of heat. It is conceived, therefore, that 

 heat is developed in three stages: (1) During process preliminary to 

 contraction; (2) during contraction; (3) during relaxation. 



The Mechanism of Muscular Activity is by no means clear. As we 

 have seen, the application of the laws of the thermodynamics to 



* The efficiency of a labourer seems to be 20-25 per cent. 



