374 THE PROPERTIES OF STRIPED MUSCLE. 



the curve under varying physical conditions, is shown. As regards temperature, 

 Z "ktioTmay be" stated'generally thus:-At 0° C the summit of the curve 

 does not rise above X, and only rises to this level slowly, whereas above 

 0° C the height of the summit increases with the temperature, as does also the 

 rapidity with which it attains this height. Kaiser therefore concludes that 

 there is a certain limit in the response of a muscle to a single instantaneous 

 excitation, at which the contractile stress which causes shortening ceases to 

 act • and he believes that any further diminution of length is due to the 

 velocity imparted to the process of shortening by this stress, and varies 

 accordingly. 



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Fig. 206. — Isotonic curves of muscle so loaded 

 that its summit is in the second equili- 

 brium position, in the one case (con- 

 tinuous line) unloaded on the summit. 

 I, line indicating equilibrium length of 

 muscle at rest when unloaded ; g, the 

 length when loaded. — After Kaiser. 



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Fig. 207. — Isotonic curve with the 

 same muscle more heavily loaded 

 to begin with, and unloaded on 

 the summit, g', line indicating 

 length of muscle when loaded. 

 — After Kaiser. 



To complete the evidence that this limit is that of the excitatory equilibrium 

 position of the muscle {i.e. that it is A), Kaiser seeks to demonstrate that it 

 is constant under other varying physical conditions besides temperature. He 

 has given satisfactory proof that this is so in the case of load, which, after 

 temperature, is the physical condition of most importance in determining the 

 " form " of an isotonic contraction. His method consisted in unloading 

 differently weighted muscles at different points on their course, whereby 

 he found that it is only when the muscle is unloaded at the moment that it 

 is at the length A, as ascertained from the isotonic curve of an unweighted 



Fig. 208.— Series of release curves following arrest, with muscle cooled to 0° C. 



muscle, that the free end of the muscle returns at once to _ its original 

 unexeited level, /. Two of Kaiser's curves showing this are given in Figs. 206 

 and 207. In the first experiment (Fig. 206) the muscle was so loaded that the 

 summit of the curve was at the level of A ; it was relieved of its load at the 

 moment of culmination of the curve, and it is seen that it relaxes at once to 

 its natural length, I — apparently indicating that there was no more available 

 contractile stress at the moment when unloading took place. In the experi- 

 ment to which Fig. 207 refers, the weight on the muscle was heavier, so that 

 the summit of the curve was below the level of A. It was again relieved of 

 its load at the moment of the culmination of the curve, and the occurrence of 



