112 Dr. J. A. Mc William. The Structure and Rhythm [Jan. 29, 



ventricular contraction may occur in sequence to a sinus beat without 

 the intervention of a contraction of the proper auricular tissue at all. 

 This is evidenced by the state of matters observable in a mode of 

 cardiac action which frequently presents itself as a result of certain 

 conditions — especially nervous influences — to be afterwards described. 

 The phase to which I refer is that in which contraction of the sinus, 

 followed by contraction of the ventricle, occurs once or many times 

 without any auricular contraction at all. Here the contraction is first 

 seen in the sinus ; after an appreciable pause it passes over the 

 ventricle, the whole auricle meanwhile remaining perfectly motionless. 

 Moreover, if the ventricle is made to contract first (e.g., by direct 

 stimulation), the contraction is after a short interval propagated 

 to the sinus, the auricle being perfectly quiescent as before. And the 

 presence of the proper auricular tissue is not at all necessary for the 

 transmission of the beat between sinus and ventricle. For the whole 

 of the auricle proper can be removed without interfering with the 

 propagation of the contraction from the sinus to the ventricle — pro- 

 vided the direct anatomical connexion of the sinus and ventricle be left 

 intact — that connexion which has been described as a prolongation of 

 the ventral wall of the sinus to the ventricle (basal wall) . In the 

 absence of all the auricular tissue the ventricle continues to respond 

 regularly to each beat of the sinus. That the ventricular beats here 

 observed are really consequent on the preceding sinus beats — that the 

 action of the ventricle is not an independent automatism, can be 

 readily shown by detaching the ventricle from all connexion with the 

 sinus tissue. When such is done the ventricle either stands still or 

 goes on beating at a much slower rate than the sinus, and with a 

 rhythm independent of that of the sinus. 



It is obvious then that contraction may readily pass from sinus to 

 ventricle, or from ventricle to sinus, without the intervention of any 

 of the proper auricular tissue. And this condition seems to obtain 

 not only in the heart of the eel but in that of many other fishes as 

 well, e.g., salmon, carp. 



On the other hand, contraction can be readily propagated from the 

 sinus to the ventricle, and vice versa, through the auricular tissue, 

 without the presence of the direct anatomical connexion (basal wall) 

 between sinus and ventricle. For if this connexion be completely 

 divided it will be found that the contraction can pass with the greatest 

 regularity from the sinus over the intact auricle to the ventricle. 

 And partial section of the auricle shows that a very slender strip of 

 auricular tissue is sufficient to allow the transmission of the contrac- 

 tion to take place. A " blocking " of the contraction can easily be 

 brought about by further section or by carefully applied pressure — 

 just as Gaskell has found to be the case in the heart of the tortoise. 

 And many of the conditions which Graskell has described with refer- 



