98 



ELECTRO-PHYSIOLOGY 



CHA1>. 



approximate constancy of steepness in the descending portion. 

 Yet this constancy relates only to a given upper portion of the 

 curve. The final return to equilibrium occurs more and more 

 slowly with decrease of temperature (and contraction residue). 



There is a marked difference in the effect of cold, and of 

 fatigue, with regard to the time-relations of muscular contraction : 

 on cooling, the descending portion of the curve is as steep as, or 

 steeper than, the ascending portion ; but in fatigue, which equally 

 prolongs the contraction-process, it is found by all authors to be 

 less steep. 



A second conspicuous effect, overlooked in earlier researches, 

 is the rise in height of the contractions, visible within a certain 

 range, on cooling. The lift shows an absolute minimum near the 

 freezing-point (of muscle-substance), where no further alteration 



FIG. 45. Schematic representation of isotonic curves of contraction at different temperatures. 



(-5 to + 42iC.) (J. Ga.l.) 



in height can be observed on stimulation, and a relative mini- 

 mum at about 19 C., from which point it rises to the alxu/utr 

 maximum at about 30 C., and the relative maximum at Q J C. 

 The minimum duration of contraction coincides with the absolute 

 maximum of lift, and increases constantly from this point, with 

 falling temperature, until the contraction disappears. The latent 

 period behaves like the period of contraction, increasing con- 

 stantly with falling temperature. We have said that an absolute 

 maximum of twitch was reached at about 30 C. ; if the tempera- 

 ture rises beyond this, excitability and height of lift decrease 

 more and more, while the duration of contraction remains 

 approximately equal (Fig. 45, A). 



At a moderate rate of heating it is possible to show that the 

 excitability of muscle to electrical stimuli disappears almost 

 entirely before the appearance of contraction from heat rigor. 

 With the isometric method temperature of course produces the 



