THE THERMAL RESPONSE. 



405 



exactly the same way as an elastic cord, the curve drawn when the slider was 

 moving from to 10 would be identical with that drawn during the return 

 movement from 10 to 0. The curves (Fig. 221) show that it is not so. 

 With diminishing load the writer follows the line a b ; with increasing load, 

 the line V a', or rather b a' ; nor does it make any difference (with the excep- 

 tion indicated in the description of the figure) which of the two curves is 

 drawn first. Now, as was explained in p. 357, every extension curve serves as 

 a " work diagram." The area included between the line b a, the vertical line 

 b e, and the horizontal line e a, represents the amount of positive work done by 

 the muscle in contracting when carrying a diminishing load. Similarly, the area 

 limited by the lines b a, b e, e a', expresses on the same scale the negative work 

 done by the weight on the muscle in the act of 

 extension. As indicated by the figure, this area 

 is always larger than the other, exceeding it by 

 the area enclosed between a a and the two curves 

 drawn by the muscle in extension and in shorten- 

 ing respectively. Consequently, in order to get 

 at the true amount of heat attributable to the 

 chemical change which has taken place in the 

 muscle during the period of observation, the 

 amount of heat corresponding to the excess of 

 the negative over the positive area must be 

 deducted from the total quantity indicated by 

 the increase of temperature. 



As has been already stated, it makes no differ- 

 ence as regards the amount of work done, whether 



the load first increases then diminishes, or first FIG. 221. Two 



diminishes then increases ; but, as regards the 

 total production of heat, the experiments yielded 

 a very remarkable though not unexpected result, 

 for it was found that the heat-effect is nearly 

 twice as great when the muscle is allowed to 

 do external work during the first half of the period 

 of excitation, than in the contrary case. The 

 meaning of this fact will be better understood if 

 we first refer to the results of preliminary experi- 

 ments, in which the heat production was measured 

 of a muscle tetanised for equal times, e.g., 1*9 

 seconds, in one case (^4) with diminishing, in 

 another (B) with increasing, load. In A the total 

 amount of heat produced, as indicated by the rise 

 of temperature, was 123*4 microcalories, and the 

 heat equivalent of the positive work done during 

 the period of shortening was 18*3 microcalories. 

 Hence the total heat product of the oxidation 

 which took place during the same period, was 131'7 ( = h + wh). In B the 

 total thermogenesis was 118*5 microcalories, and the heat equivalent of the 

 negative work done 32*3. The heat value of the material oxidised was therefore 

 86*2 (h - wh). The chemical process was thus more active when the work done 

 was positive than when it was negative, in the proportion of ^f- . In the form 

 of experiment (C) in which the thermogenetic effects of two excitations of equal 

 duration are compared, in one of which (a after b, i.e. b a) & diminishing load 

 is preceded, in the other (b after a, i.e. a b) is followed, by an increasing one, 

 there was a very considerable difference between b a and a b. The measure- 

 ments in microcalories were when b followed a, 137, when b preceded a, 92. 

 As in each case the amount of heat produced by stretching, namely 19 micro- 



extension 

 curves of the same excited 

 muscle taken continuously 

 with decreasing (a b) and 

 increasing (b a'} loads. 

 The figure is modified from 

 the original (Fick, loc. cit. , 

 Fig. 6, p. 550), by the 

 addition of the dotted line 

 which indicates the course 

 the writer must have actu- 

 ally followed in the case 

 described. When the order 

 of the experiment is re- 

 versed, so that the increase 

 of load precedes the 

 decrease, the tracing 

 follows the continuous 

 line e b' a' a b. The part 

 of the tracing above b is 

 no doubt chiefly due to 

 the inertia of the lever. 



