200 PHYSIOLOGY 



to consider the changes which occur in the muscle between the application 

 of the stimulus and the moment at which the first mechanical change 

 makes its appearance. 



The relaxation of muscle is helped by a moderate load, and in a normal 

 condition is complete. It is not active that is to say, is not due to a con- 



FIG. 59. 



V. Kries' apparatus for taki 

 traction 



' after-loading ' and ' arrested 

 rves.' 



traction in the transverse direction but is a passive effect of extension and 

 elastic rebound. This may be shown by allowing a muscle to contract while 

 floating on mercury. The subsequent lengthening on relaxation is very 

 incomplete. 



Even with the most careful arrangements for securing isotonicity in the 

 record of the contraction there is probably a certain amount of over-shoot 

 of the lever whenever, as at high temperatures, the contraction is sufficiently 

 rapid. The effect of this is that one cannot assume the existence of an actual 

 pull on the lever during the 

 whole time of the ascent of the 

 latter. We can therefore speak 

 of a period during which there 

 is contractile stress that is to 

 say, when the muscle is actually 

 pulling on the lever, which will 

 occupy only a part of the 

 ascent of the curve. The dura- 

 tion of this period of contractile 

 stress may be shown by re- 

 cording what is known as * arrested ' contractions. One mechanism 

 for this purpose is shown in the figure (Fig. 59). The stop Su is used 



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FIG. 60. Curves of isotonic and arrested contractions 

 of an unloaded muscle. (KAISER.) 



