122 Dr. J. A. McWilliam. The Structure and Rhythm [Jan. 29, 



any movement was perceptible in the auricle. I have not seen a 

 distinct division of the sinus into ostial and interjugular parts in 

 any fish except the eel. 



During the inhibitory standstill the condition of the various parts 

 of the heart, as evidenced by the results of direct stimulation, is 

 strikingly different from that observed in the hearts of those animals 

 in which cardiac inhibition has been chiefly studied. 



In the frog-heart, for example, a single direct excitation applied to 

 the heart during the inhibited phase produces a single beat ; the con- 

 traction begins at the stimulated point and extends over the whole 

 organ. And this result is obtained whether auricle or ventricle is 

 stimulated. In the eel's heart the state of matters is more complex. 

 The ventricle resembles the frog-heart in giving a single beat in 

 response to a single excitation ; its irritability and contraction force 

 seem to be quite unaffected. The interjugular part, except when the 

 heart is most powerfully inhibited, remains excitable to direct stimu- 

 lation. And contraction excited in the interjugular part is usually 

 propagated to the ventricle, whilst the auricle remains perfectly 

 quiescent ; contraction excited in the ventricle is commonly propagated 

 in a similar fashion to the interjugular part. The ostial part of the 

 sinus is, during strong inhibition, quite inexcitable to direct stimuli 

 of all kinds, and so is the auricle. The result of nerve stimulation has 

 evidently in this instance been the peculiar one of temporarily abolish- 

 ing the irritability of the muscular tissue. Such a result is quite 

 incompatible with the old conception of vagal action, viz., that the 

 vagus inhibited the motor discharges from the cardiac ganglia, and 

 so brought the muscular tissue into a state of quiescence. If such 

 were the case the muscle ought of course to respond readily to direct 

 stimulation. And this the auricular muscle signally fails to do. 



It has been mentioned that during the inhibited state direct stimu- 

 lation of the ventricle readily causes contraction of that part ; the 

 contraction originates at the stimulated point and spreads over the 

 rest of the ventricle. It does not pass over the auricle. The non- 

 participation of the auricle in the contraction cannot be explained by 

 the assumption of a block between the auricle and ventricle preventing 

 the contraction being propagated. For if such were the case, if the 

 auricle remained quiescent merely because the contraction failed to 

 reach it, the auricular muscle ought to respond readily to direct 

 stimulation. But it has been shown that the auricular muscle is 

 quite inexcitable, and that it is in such a state as to be quite incapable 

 of responding to any contraction impulses that might reach it. 



