THE MECHANICAL CHANGES OF MUSCLE 225 



series of contractions such as those shown in Fig. 59. It will be seen 

 that at the points x ', x", and x'" the muscle was still pulling on the 

 lever, and therefore held it up against the stop. At the point X the 

 arrested twitch returns rapidly to the base line, showing that the 

 movement of the lever in the unarrested curve above this point was 

 due to the inertia of the moving parts and not to the actual pull of 

 the muscle. 



PROPAGATION OF CONTRACTION. THE CONTRACTION 



WAVE 



The whole muscle does not as a rule contract simultaneously. 

 When excited from its nerve the contraction begins at the end-plates 



FIG. 60. Diagram of arrangement for recording the contraction wave in a 

 curarised sartorius. 



and spreads in both directions through the muscle. The rate of 

 propagation of the contraction wave can only be measured by employing 

 a curarised muscle, so as to avoid the wide spreading of the excitatory 

 change by means of the intra-muscular nerve- endings. For this 

 purpose a curarised sartorius muscle is taken, stimulated at one end, 

 and the thickening of the muscle recorded by means of two levers 

 placed, one near the exciting electrodes and the second at the other 

 end of the muscle, as shown in the diagram (Fig. 60). The difference 

 between the latent periods of the two curves represents the time taken 

 by the contraction wave in travelling from a to b. By measurements 

 carried out in this way it is found that the rate of propagation of the 

 contraction in frog's muscle is 3 to 4 metres per second ; in the muscle 

 of warm-blooded animals it may amount to 6 metres. 



15 



