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



snch circumstances the ventricular action is often completely sus- 

 pended until such time as the auricular contractions are slower and 

 consequently stronger. 



When failure of the ventricular sequence has occurred, the normal 

 order of events can often be restored for a time — (1) by applica- 

 tion of salt solution to the junction of the ventricle with the rest of 

 the heart; (2) by passage of an interrupted current through the 

 same part ; or (3) by stimulation of the vagus nerve, leading to car- 

 diac standstill, followed by a phase during which the normal ventri- 

 cular sequence is restored. Whether this result of vagus stimulation 

 is due to a direct beneficial effect on the tissues or to the rest which 

 has been afforded to the various parts, is not at present clear. 



Failure of the ventricular sequence seems to be at times associated 

 with a depressed excitability of the ventricle itself. 



All the parts of the heart when isolated from one another can mani- 

 fest the property of automatic rhythmical action, though this property 

 is possessed by the different portions of the organ in very different 

 degrees. The excised ostial parts of the sinus go on beating at the 

 ordinary rate of the heart's rhythm. The interjugular part when 

 isolated exhibits (after a short pause) an independent rhythm which is 

 slower than that of the ostial parts. The prolongation of the sinus to 

 the ventricle comes next with regard to rhythmic power ; then the 

 auricle ; and finally the ventricle. As regards the property of inde- 

 pendent rhythmic contraction then, the various parts of the heart 

 form a descending series, the highest term of which is the ostial 

 part of the sinus ; the lowest term is the ventricle. 



The high rhythmic power possessed by the sinus constitutes it the 

 leader in the series of events that make up the cardiac beat. Each 

 contraction originating in the sinus of a normal intact heart spreads 

 over the remaining parts of the organ, and thus leads to a rate of 

 action in these parts identical with that in the sinus — a rate of action 

 very much more rapid than they could exhibit in virtue of their own 

 independent rhythmic power. 



III. Some Points in the Behaviour of the Heart with regard to the 

 Results of Direct Stimulation. 



I shall here deal chiefly with the phenomena exhibited by the 

 ventricle, reserving the consideration of the results obtained in the 

 case of the auricle until the influence of the cardiac nerves has been 

 discussed. 



A single stimulation applied to the ventricle is followed by a 

 single responsive beat. If the ventricle experimented on forms part 

 of an intact heart, the result of a single excitation is to cause a re- 

 versed beat of the heart, the ventricle contracting first. And if the 



