THE MECHANICAL RESPONSE. 



373 



care is taken to avoid the slightest change of position of the fixed end, so that 

 the graphic record of each length, due to extension by the load, may be abso- 

 lutely reliable. 



The first step is to determine and record graphically in the unweighted 

 muscle its "natural" length when 

 excited by a maximal stimulus. If 

 we call this length A we may designate 

 the horizontal line corresponding to 

 it by the same letter. The position 

 of that line is determined as follows : 

 — An ordinary isotonic curve having £ 



been drawn with an " unloaded " 

 muscle, a series of arrest curves are 

 taken from the same starting-point, 

 as shown in the figure (Fig. 204), the 

 stop being slightly raised after each 

 excitation. At first the lever hesitates 

 at the stop for several hundredths of 

 a second ; but, as the succeeding 

 curves show, the period of hesitation gets less and less, until finally the 

 lever falls immediately. The same thing happens, whatever the tempera- 

 ture of the muscle; and if the experiment is repeated with the same un- 

 weighted muscle, at temperatures, say, of 0°, 14°, and 30° C, it is found that 



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FlG. 204. — Curves of isotonic and arrested 

 contractions of an unloaded muscle, with 

 maximal stimulation at ordinary tem- 

 perature, x' , x", x'" , and X, points at 

 which the lever was arrested in successive 

 curves. — After Kaiser. 



Fig. 205. 



(1) Isotonic curve of unloaded muscle at temperature 14° C. ; (2) the 

 same, at 25° C. ; (3) at 0° C. ; (4) at 14° C, with load on muscle. 

 a b indicates level of fixed end of muscle. I indicates level of free 

 end of muscle in its first equilibrium position (i.e. when un- 

 loaded and at rest) ; x denotes the length of muscle in its second 

 equilibrium position (i.e. that which it tends to assume when un- 

 loaded, in response to a single maximal stimulation) ; g, line repre- 

 senting length of muscle when so loaded that the summit of the 

 curve is at x ; x, x' , point at which the lever falls at once on 

 arrest, in curves drawn with muscle at 14° and 25° C. respect- 

 ively ; x° and x", summit of curves drawn with muscle at 0° C. 

 and when loaded respectively. The marks on the A line indicate 

 hundredths of a second. — After Kaiser. 



the height at which there is no hesitation of the lever is the same for each, 

 however much the height of the summit of the curve, and its duration may 

 vary. A horizontal line at this height, is therefore the one required. It is 

 represented in the diagram (Fig. 205), in which its relation to the summit of 



