SECTION VIII 



THE INTIMATE NATURE OF MUSCULAR 

 CONTRACTION 



EXPERIMENTS on the metabolism of the body as a whole show 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 portion 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 heat engine is expressed by 



T T 1 



the formula E = ~ , where T is the highest temperature (in absolute 



measurement) obtained by the working substance and T 1 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 temperature 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 transforma- 

 tion .of heat into mechanical energy. He has found that non-living sub- 

 stances, 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 

 the platinum coil. He imagined that the chemical changes in the muscle 

 liberate heat and that the effect of this heat upon the doubly refractive 

 particles is to make them imbibe the surrounding water so that they change 



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