THE PULSE. 529 



undoubtedly, the contraction of the auricle, but in order to 

 locate this wave or, indeed, to interpret at all the complicated 

 venous pulse, it is necessary to have a simultaneous tracing of 

 the arterial pulse, preferably the carotid, or of the apex beat of 

 the heart. Either of these latter tracings enables one to mark 

 upon the venous pulse the point at which the ventricular systole 

 begins, and the wave immediately preceding this point must 

 be due to the auricular contraction, the a wave (Figs. 217 and 

 218). Following the rise of the a wave there is a fall, the first 

 negative wave, which is due to the auricular relaxation. The 

 interpretation of the other two positive and negative waves 

 has been the subject of much discussion. Mackenzie, one of 

 whose tracings is reproduced in Fig. 217), believed that the 





Carotid 



R*. Int. Jugular 



Fig. 217. Simultaneous tracings of the carotid and venous pulses. In the venous 

 tracing (internal jugular) a indicates the auricular wave due to the contraction of the auri- 

 cle; c is the carotid wave due (Mackenzie) to an impulse from the neighboring carotid 

 artery; v is the ventricular wave due to the checking or stagnation of the flow into the 

 auricle as this chamber fills during the period of closure of the auriculoventricular valves; 

 x, dilatation due to auricular relaxation; y, the period of ventricular diastole. (Mackenzie.) 



c wave is due simply to the pulse in the neighboring carotid 

 artery, and that, therefore, it has no significance in regard to 

 changes within the heart itself. Careful records made by other 

 observers show, however, that this explanation is insufficient. 

 The c wave begins in the jugular before the arterial pulse wave 

 reaches the carotid, hence this wave cannot be due wholly to 

 the carotid pulse. As is shown in the tracing given in Fig. 218, 

 the c wave begins, in fact, at the very moment of ventricular 

 systole. The explanation of it which meets with most accept- 

 ance is that it is due to a sharp protrusion of the auriculo- 

 ventricular valves into the cavity of the auricle. At the begin- 

 ning of the ventricular systole these latter valves are in position 

 for closure, while the semilunar valves at the opening to the 

 pulmonary artery are tightly closed. For a short period the ven- 

 tricular muscle contracts upon a closed cavity, and the pressure 

 upon the contents rises rapidly. It is at the beginning of this 

 brief period that the auriculoventricular valves are protruded 

 34 



