250 PHYSIOLOGY 



is made to contract against a strong spring, so that it cannot shorten 

 at all (isometric contraction). 



In view of the comparison of the muscle to a heat engine, it becomes 

 interesting to inquire into its efficiency, i.e. the relation of the work 

 to the total energy expended. This amount is found to vary within 

 very wide limits. In a fresh muscle the heat energy may be twenty- 

 five times as great as the work energy, but the heat evolved with 

 each contraction diminishes with fatigue more rapidly than the work 

 done, so that the proportion may fall to as low as three to one. In 

 the intact animal, in the dog fed on a pure flesh diet, Pfliiger has 

 calculated that the efficiency may be as great as 48 per cent. The 

 experiments made by Atwater and Benedict on man point to a 

 mechanical efficiency of about 12 to 20 per cent. The efficiency 

 of a heat engine is determined by the difference of absolute tempera- 

 tures obtaining on the two sides of the machine ; and since we cannot 

 imagine even minutely localised changes of temperature in the animal 

 body of more than a few degrees Centigrade, we must discard altogether 

 the analogy of the steam-engine, and seek some other explanation of 

 the mechanism by which the muscle is enabled to transmute the chemical 

 energy of its food into work or heat. It seems probable that the two 

 products, heat and work, are simultaneous and independent in their 

 origin, and that any proportion between them, therefore, is accidental. 

 The muscle is, in fact, not a heat engine, but a chemical engine. 



This conclusion is borne out by the observations of A. V. Hill. He 

 finds that the heat production of a single twitch is usually of very 

 short duration, i.e. less than O'l sec., but that it is prolonged by 

 depriving the muscle of oxygen, and may outlast the mechanical 

 effect. No constant ratio is found between work done and heat 

 evolved. The important factor is the tension, as first pointed out 



by Heidenhaim. In an isometric muscle twitch ({ e . enslonN \ - g 



H \ ' heat / 



constant whatever the number of fibres contracting, the strength 



T 



of contraction, or the initial resting tension on the muscle ; i.e. 



H 



is the same for every fibre. The tension is the measure of the potential 

 energy which is thrown suddenly into being at the moment of excita- 

 tion, and this is proportional to the heat, and therefore to the total 

 chemical change evoked. On the basis of these results, Hill suggests 

 that there are three stages in the process of muscular contraction : 



(i) The liberation of certain molecules following an excitation. 



(ii) The action of these molecules on certain local structures, in 

 producing a local tension. 



(iii) The removal or replacement of these molecules, under the 

 action of oxygen, with evolution of heat. , 



