1040 PHYSIOLOGY 



before it has been completed in the aorta. Fig. 415 represents a pulse- 

 curve taken from the radial artery. The elevation due to the expan- 

 sion of the artery is rapid and uninterrupted. We have already 

 explained that this is due to the sudden pumping of blood into the first 

 part of the aorta, whence the impulse is transmitted as a wave along 

 the arteries. The curve descends gradually till the next beat occurs, 

 since the elastic reaction of the arteries, which tends to keep up the 

 pressure, acts more constantly and steadily than the heart-beat. On 

 this descending part of the curve occur two or three secondary eleva- 

 tions : b is the primary or ' percussion ' wave, c the pre-dicrotic or 

 ' tidal ' wave, and e the dicrotic wave. Elevations may occur on the 

 curve after e which are called post-dicrotic waves. It is better to 

 class the elevations before the dicrotic notch d as systolic elevations, 

 and those afterwards, including the dicrotic elevation itself, as diastolic. 



FIG. 415. Pulse-curve from radial artery. 



For the exact understanding of these elevations it is necessary to take 

 simultaneous tracings of the pressures in the left ventricle and in the 

 aorta (Figs. 416, 417). In this way we may dissociate the waves caused 

 by the ventricular systole from those having their origin in the aorta. 

 In Fig. 416 are represented typical tracings of cardiogram, intraventri- 

 cular pressure, and aortic pressure, taken simultaneously. The dotted 

 lines are drawn through synchronous parts of the curve. Considering 

 first the dotted part of curve II and curve IV,* we see that the contrac- 

 tion of the ventricle begins at A ; the rise of intra ventricular pressure 

 from A to B is without effect on the aortic pulse ; at B the intra- 

 ventricular is exactly equal to the aortic pressure, and then rapidly 

 rises above it. Since the aortic valves offer no resistance to the flow of 

 blood from ventricles to aorta, they must open so soon as the intra- 

 ventricular exceeds the aortic pressure, and this is shown to be the case 

 by the rise of pressure in the aorta. From B to c the ventricle is still 

 contracting and forcing the blood into the already distended aorta, 

 so causing a rise of pressure. At c the ventricle relaxes, the intra- 

 ventricular pressure falls quickly, and at D has fallen below the aortic 

 pressure. The aortic valves must now close, since the pressure is 

 greater on their aortic side. The fall of pressure on the ventricle now 



* Curve IV in Pig. 410 must be compared with the pulse tracing from the 

 radial artery in Fig. 415. It will be seen that, apart from the fact that Fig. 410 

 IV is more lengthened out than Fig. 415 owing to the great rapidity of the 

 recording apparatus, the curves are practically similar. 



