

means of recording the movements of the left auricle. As a 

 routine practice the method is naturally impossible, and con- 

 traction of the left auricle is also frequently represented on the 

 cardiographic curve. In this connection it is of interest to notice 

 that the auricular wave in the cardiogram often shows a shortened 

 duration, as does that in the oesophageal, when compared to that 

 in the jugular. 



The evidence so far considered has led to certain conclusions 

 as to the time relations of the events of the cardiac cycle in the 

 normal human subject, and these conclusions have been incor- 

 porated as carefully as possible in the figure given on page 88. 

 The subsequent discussion will be based to a large extent on the 

 assumption that the events are correctly represented. That they 

 are approximately correct is beyond doubt, but that they are 

 rigidly correct is perhaps improbable. The diagram represents 

 the conclusions which the evidence we possess to-day appears to 

 warrant. 



III. THE INTERPRETATION OF THE AURICULAR PRESSURE 

 CURVE AND THE JUGULAR PULSE TRACING 



In passing to a consideration of the events involved in the 

 production of the three main waves and three main depressions 

 of the tracings, it will first be convenient to examine the ultimate 

 factors to which each in its turn is due, whether it occurs on the 

 jugular, oesophageal, cardiographic, or auricular curve, and then 

 to briefly notice, where necessary, the modifications which occur 

 in the jugular tracing by the interposition of interfering factors. 



(a) The First Positive or "a" Wave. The presystolic 

 onset of this wave leaves little doubt that it is due to the con- 

 traction of the auricle, and it is universally attributed to this 

 cause. According to Fredericq ( 12 *' c ) it disappears from the 

 tracing, when as a result of tetanisation of the auricle, this 

 chamber ceases to beat. 1 In cases of partial heart block, in 

 which the auricles maintain a rhythm which is a multiple of the 



before the left ; that on the other hand the left ventricle contracts '03 to '04 MT. 

 before the right ; and that the delay between the onset of auricular and ventricular 

 systole is '08 to *10 sec. Schmidt-Nielsen's figures (**) for the delay from right to 

 left auricle agree with those of Stassen. ComjMire Fredericq's remarks (**). 

 1 Heriug ( ac ) has confirmed this result by the use of vagal inhibition. 



