i GENERAL PHYSIOLOGY OF MUSCLE 65 



a fresh muscle and a fatigued muscle, the former develops more 

 heat than the latter, as if the chemical activity necessary for 

 developing the same amount of useful external work were greater 

 in the fresh muscle and less in the fatigued. In a series of 

 successive contractions of equal height, carried out by a muscle 

 loaded with the same weight, so that each contraction performs 

 the same amount of work, the development of heat diminishes 

 between the first and the last of the series. This shows that 

 fatigue can be detected in the diminished heat-production before 

 it becomes evident in the lessened height of contraction. Accord- 

 ingly, as it becomes fatigued, the muscle functions more economi- 

 cally i.e. a less amount of energy is transformed into heat. 



When the impulses that reach the muscle follow so rapidly as 

 to give rise to tetanic fusion of the contractions, the production of 

 heat increases progressively up to a certain maximum, in propor- 

 tion to the increasing height to which the weight is raised. 



The heat developed in tetanus increases with increment of the 

 load and corresponding tension of tjhe muscle. When the weight 

 is so great as to inhibit contraction altogether, more heat is 

 developed than when the load is less and the muscle can shorten 

 a little. During the development of tension the heat production 

 is greater than when the tetanic rise is complete. During a brief 

 tetanus the same amount of heat is liberated at each instant. But 

 during the contraction, and possibly during the relaxation, that 

 precede and follow tetanus, a much larger quantity of heat is 

 developed. 



Heidenhain's work on muscular thermogeuesis was extended 

 and completed by Fick and his pupils. Fick in his first experi- 

 ments (1884) resumed the study of the question already investigated 

 by Beelard. Heidenhain's discovery that the sum of the energy 

 (work and heat) developed by the muscle is proportional to its 

 tension during its activity, does not contradict Beclard's view, as 

 Hermann also pointed out, that with constant tension the sum of 

 energy developed by the muscle (work and heat) is in direct ratio 

 with the intensity and duration of its activity, so that, caeteris 

 paribiis, the energy liberated in the form of work is inversely 

 proportional to that liberated in the form of heat conformably 

 with the law of conservation of energy. 



In order to prove this theory experimentally, Fick employed 

 Heidenhain's method on the excised muscles of the frog. To 

 compare the thermal production in useful work with that of con- 

 traction by which no external work was performed, he invented 

 an ingenious apparatus which he termed " Arbeitssammler." This 

 is a small windlass which the muscle turns on contracting 'against 

 the constant resistance of a weight, which can be prevented from 

 dropping again during relaxation by putting a brake on the wheel 

 (Fig. 40). When this is applied the muscle is unloaded, i.e. freed 



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