THE THERMAL RESPONSE. 



401 



general the chemical work done by a muscle, as indicated by the 

 heat produced in contracting in response to an instantaneous stimulus, 

 is determined by the resistance offered, up to a certain limit. If this 

 limit is exceeded, the thermal effect cannot be increased, and may even 

 be diminished by increasing the load. 



Relation of thermogenetic effect to strength of stimulus. — The 

 question whether and to what extent the chemical, i.e. the thermal, 

 response is dependent on the strength of the stimulus, was investigated 

 many years ago in Heidenhain's laboratory. 1 For this purpose the 

 increase of temperature produced in lifting and letting fall a weight, 

 was measured in a series of excitations with " inadequate " stimuli of 

 graduated strengths. The difficulty of making such measurement is 

 considerable, for the strength of a maximal excitation differs so little 

 from that of an excitation which is just sufficient to evoke a measurable 

 response, that the range of excitation values which can be compared 

 experimentally is very limited. On the whole, it was found that in 

 isotonic contractions the thermal effect varies with the stimulus, in the 

 same way that the work varies ; but that, if the load remains the same, 

 the increase of heat production which results from increasing the 

 stimulus is more than proportional to the greater lift. The meaning of 

 this fact will be considered later. 



The experimental results which have been already referred to might 

 all be understood as consistent with the notion that the one condition 

 which is influential in determining the amount of the thermal response 

 to a single maximal stimulation, is tension or strain. It was, however, 

 recognised by Fick many years ago that the evidence in favour of so 

 simple a relation between tension and thermogenesis was insufficient. 

 It was therefore necessary to devise an experimental method by which 

 the question could be elucidated. If the thermal response depended 

 solely on tension, it was obvious that its amount must be greater under 

 isometric conditions than under any other. This could be tested by 

 comparing two cases, 2 in one of which (a) the muscle was under 

 isometric conditions during the whole of the mechanical response, while 

 in the other (b) it was released at the moment of maximum tension and 

 allowed to lift a weight of 20 grms. The result showed that, notwith- 

 standing that in b the tension was at once reduced at the moment of 

 release, the thermogenesis was greater than in a. The numbers, each 

 of which represents a deflection of the galvanometer, were as follows : — 



(a) No work done 



(b) Weight lifted and let fall 



etc. 



The numbers relate to successive excitations. 



By this experiment it was shown for the first time that in muscular 

 action the sum of the thermogenetic values of tension and work is 

 greater than the maximum thermogenetic value of tension by itself. 

 In our study of continuous contraction we shall see to what important 

 results this leads. 



Production of heat in tetanus. — It does not necessarily follow 

 from the facts which have been stated, that persistent contraction, 



1 J. Nawalichin, " Myotlicrniische Untersuchungen," Arch. f. d. ge$. Physiol., Bonn, 

 Bd. xiv. S. 293. 



2 Fick, " Myothermische Fragen, u.s.w.," Verhandl. d. phys.-med. Gescllsch. 

 burg, 1884, N. F., Bd. xviii. S. 12, Sep. Abd. 



VOL. II. — 26 



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