64 PHYSIOLOGY CHAP. 



and declines with its decrement. The facts collected by Heiden- 

 hain, however, show that the rise and fall of temperature in the 

 muscle are not strictly proportionate to the increase and 

 diminution of the mechanical work which it performs ; generally 

 speaking, the thermal increase is much less than the increase of 

 work. This proves that the muscle works more economically 

 when it lifts a moderate weight than when it lifts a lighter one. 



The property which the muscle possesses of adjusting the 

 quantity of energy which it develops under a constant stimulus 

 to the greater or less resistance which it has to overcome, is very 

 important. If the strength of its reaction depended only on the 

 strength of the stimulus, and was independent of the load, then 

 the development of muscular energy the nerve impulses remain- 

 ing uniform would not be in proportion with the external work 

 that had to be performed. Heidenhaiu's discovery that the total 

 sum of energy developed by the muscle depends on the degree of 

 tension due to the resistance it encounters in contracting, shows 

 that it possesses a mechanism in itself which is capable inde- 

 pendently of the nervous impulses of partially regulating its 

 intrinsic metabolism according to the needs of the moment. 



Again, when the load remains constant, and the strength of 

 stimulation is progressively increased, the development of heat 

 increases within certain limits with the height of the con- 

 tractions and the mechanical work performed, till it reaches a 

 maximum. So that the metabolism and heat production of 

 muscle are regulated not only by tension, but also by the nervous 

 system, owing to the varying intensity of the impulses which it 

 transmits to the muscle. 



It should, however, according to the results of Heidenhain and 

 Nawalichin, be observed that, just as we have seen with constant 

 stimulus and increasing load, so too with a constant load and a 

 progressively increased stimulus, the increase in heat and work 

 development are not parallel, but the maximum production of heat 

 is always reached before the maximum of work, i.e. the heat pro- 

 duction increases more rapidly than the height of the contractions. 

 This proves that the muscle works more economically whenever it 

 is more strongly excited from the nerve, and forced to do more 

 work. 



But when the same amount of work is performed by a muscle, 

 on the one hand by many small contractions, on the other by 

 fewer but larger contractions, less heat, according to Heidenhain, 

 is developed in the first case than in the second. This agrees 

 with the common observation that it is more fatiguing to ascend a 

 rapid incline with long steps, than a less steep slope of the same 

 height, with shorter steps. 



Heidenhain brought out another interesting fact which is not 

 easy to explain. When the same amount of work is performed by 



