214 A TEXTBOOK OF PHYSIOLOi-Y 



The a wave is due to the contraction of the right auricle. It 

 disappears when this chamber is not beating, and is sometimes re- 

 placed by a series of oscillations when the auricle is fibrillating. Its 

 exact mode of origin and propagation is not known, but it is certainly 

 synchronous with, and an indication of, auricular contraction. 

 Comparison of the records in Fig. 116 shows that /) precedes a. Allow- 

 ance must be made for time of propagation. 



The c wave owes its origin to the ventricular systole, since it dis- 

 appears when the ventricle ceases to beat. It has been supposed to 

 be due to the pulsation of the neighbouring artery, and in many 

 instances this is undoubtedly the case; but it occurs also in curves 

 taken when the arterial influence has been removed, and also in the 

 curves of interauricular pressure. It is possibly due to the bulging 



FIG. 116. SIMULTANEOUS RECORDS SHOWING THE TIME RELATIONS or WAVES OF 

 JUGULAR PULSE AND ELECTROCARDIOGRAM. (W. T. Ritchie, from Cowan's 

 " Diseases of the Heart.") 



P Precedes a, B precedes c. 



into the auricle of the closed tricuspid valve at the onset of ventricular 

 systole. The mode of origin is under discussion both factors may con- 

 tribute while for clinical purposes it may be reckoned as synchronous 

 with the primary wave of the arterial pulse in the neck. In Fig. 116 

 it is seen that the wave 7? of the electrocardiogram precedes the wave c. 



Various divergent views are held about the origin of the v wave. 

 The two views generally held are (1) that it is due to the filling of 

 the auricles during ventricular systole; (2) that it is due to the move- 

 ment of the auriculo-ventricular groove at the beginning of diastole 

 (Fig. ~>9), Probably both factors contribute. 



The depressions on either side of the c wave, sometimes called 

 x and x', are really one depression broken by the c wave. They are 

 probably due to (1) the auricular relaxation dependent upon the 

 inspiratory diminution of intrapleural pressure; (2) the expansion of 

 the auricular walls which results from ventricular systole, by the de- 

 pression of the diaphragm formed at the floor of the auricle by the 

 closed auriculo-ventricular valves. The first factor is active only 

 during inspiration. 



