SECTION VIII 



THE INTIMATE NATURE OF MUSCULAR 

 CONTRACTION 



EXPERIMENTS on the metabolism of the body as a whole sh( ^ 

 that the energy of muscular work is derived from the oxidation of 

 the food-stuffs. In man the performance of work involves an increase 

 of the oxidative processes of the body with a corresponding evolution 

 of energy, of which four-fifths will appear as heat while one-fifth 

 may be transformed into mechanical work. In this respect the 

 physiological mechanisms for the production of mechanical energy 

 resemble the greater number of the machines employed by man for 

 the same purpose. In nearly all these the prime source of energy 

 is the oxidation of carbon and hydrogen in the form of coal or oil. 

 In the steam-engine and internal-combustion engine the whole energy 

 set free by the process of oxidation appears first as heat, and then a 

 certain proportion of the heat is converted into mechanical work. 

 There is a limit to the efficiency of such heat engines, depending 

 on the maximum differences of temperature available between the 

 two sides of the working part of the machine. The efficiency of any 



T T' 



heat engine is expressed by the formula E = , where T is 



the highest temperature (in absolute measurement) obtained by 

 the working substance and T 7 is the lowest temperature of the same 

 substance. Ordinary engines rarely attain more than half this ideal 

 efficiency, but it is evident that the greater the difference of tem- 

 perature available the greater will be the efficiency of the machine. 

 Internal-combustion engines, such as the gas-engine or the oil-engine, 

 therefore give a greater percentage of the total energy of the fuel 

 out as mechanical energy than is the case with the steam-engine. 



Engelmann has maintained that in muscle there is a similar 

 transformation of heat into mechanical energy. He has found that 

 non-living substances, which contain doubly refractive particles and 

 possess the property of imbibition (e.g. catgut) when soaked with 

 water, will contract on heating and relax again on cooling. He 

 has constructed a model in which a thread of catgut in water, 

 surrounded by a platinum coil, can be made to simulate muscular 

 contractions and relaxations by passing a heating current through 



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