THE SECOND STANNIUS LIGATURE 129 



experiment, opinions differ. One view is that the first ligature causes 

 standstill of auricles and ventricle by stimulating the inhibitory 

 mechanism at the junction of sinus and auricles, and that this inhibi- 

 tion lasts some time, until the mechanism becoming fatigued, the 

 inhibited parts gradually regain a rhythmic contraction. This view 

 is borne out by repeating the experiment upon a heart poisoned by 

 atropine. The atropine paralyses the inhibitory nerve terminals of 

 the heart, and it is found that standstill is impeded or prevented by 

 atropine. The second ligature is supposed to act by cutting off the 

 inhibitory influences, set in action by the first ligature, from the ven- 

 tricle, which then commences to beat at its own rhythm. 



According to Gaskell, the probable explanation of the experiment is 

 that the first ligature blocks the contraction wave originating in the 

 sinus. Auricles and ventricle, therefore, for a time remain quiescent, 

 ultimately originating a rhythm of their own, though this requires 

 time. This does not, however, thoroughly explain all the facts, for if 

 the auricles and ventricle be excised, they gain a fresh rhythm in 

 quite a short time, and it therefore seems to follow that the first 

 ligature must be causing some inhibitory influences preventing the 

 establishment of that rhythm. On Gaskell's view the action of the 

 second ligature is to stimulate the ventricle, and consequently to lead 

 to a rhythmic contraction. It is difficult to see why the first ligature 

 should not also act as a stimulus, or why the second should not 

 stimulate the auricle rather than the ventricle. 



