400 THE PROPERTIES OF STRIPED MUSCLE. 



it is estimated from recent experimental data, that the heat produced 

 during a period of active muscular exertion in the whole body is about 

 five times as great as the heat-equivalent of the external work done ; 

 so that the work-heat is to the heat-equivalent of the total chemical 

 oxidation of the body as 1 to 6. Assuming this estimate to be correct, 

 about three-fifths of the total heat set free in the body during 



H— Wh 



exercise, must be produced in the muscles {i.e. the quotient " ~ W1 



is, in muscle 3, in the whole body 5), provided always that the propor- 

 tion above given, 1 to 4, holds for mammalian muscle. 1 



Eecurring to the first and third of the three cases referred to at the 

 beginning of the last section, p. 394, we find that whether, as in Case 1, the 

 muscle is prevented from shortening when stimulated, or, as in Case 3, is 

 allowed to lift a weight which falls when the effort ceases, the whole 

 of the chemical work done is represented by its equivalent in heat, 

 and indicated by the rise of temperature consequent on the excitation. 

 When a muscle is subjected to instantaneous stimulation, of which the 

 intensity exceeds the maximum, no corresponding increase of the 

 response is observed, and hence it might be inferred that the effect 

 of such a stimulation is to " explode " the whole of the oxidisable 

 material which the muscle at the time has at its disposal. The 

 most important result of Heidenhain's researches was the proof which 

 they afforded that this is not the case, 2 and that, although we 

 are quite right in regarding the muscular response as a process 

 of discharge, the amount of chemical potential energy liberated by 

 excitation varies, even when the strength of the stimulus remains the 

 same, according to the circumstances under which the discharge 

 takes place. Of these circumstances resistance is the most influential. 

 This is plainly shown when the thermal responses, in a series of 

 excitations in which the muscle is allowed to lift and let fall different 

 weights — say, 30, 60, and 90 grms. — are compared with each other. 

 It is found that the quantity of heat produced is not the same in the 

 three experiments, as we should expect it to be if it were dependent 

 on the amount of available energy in store, but increases with the 

 quantity of work done in each lift of the weight. It does not, 

 however, increase in the same proportion with the work done, for 

 Heidenhain found in all his experiments 3 that in maximal stimulation 

 the ratio of heat produced to work done diminishes as the quantity of 

 work done increases. In other words, the muscle works more 

 economically when moderately loaded than when lifting a very small 

 weight. 



It was further found that when a muscle is called upon to make 

 repeated efforts in lifting and letting fall the same weight, the 

 exhaustion thereby produced tells much more rapidly on thermogenesis 

 than on working power. The exhausted muscle does less work, but 

 does it more economically. In like manner it appeared that when a 

 muscle is " after-loaded," 4 so that, contracting at first without obstacle 

 it encounters the resistance subsequently, the effect of a given load on 

 heat production is not so great as it would be if the muscle were loaded 

 in the ordinary way. From these data Heidenhain concluded, that in 



1 See Gad and Heyman's " Lehrbuch," S. 504. 



2 Heidenhain, loc. cit., S. 85. 3 Heidenhain, loc. cit., S. 88. 

 4 Heidenhain, loc. cit., S. 103. 



