THE PULSE. 525 



approximately at the middle of the descent (D) and is designated 

 as the dicrotic wave. A less conspicuous wave between it and the 

 apex of the pulse wave is known usually as the predicrotic wave, P, 

 while the wave or waves following the dicrotic are designated as 

 postdicrotic. These catacrotic waves are too small, under normal 

 conditions, to be felt by the finger. Under certain abnormal 

 conditions, however, which cause a low blood-pressure without 

 marked diminution in the heart beat, the dicrotic wave is empha- 

 sized and may be detected by the finger. A pulse of this kind is 

 known as a dicrotic pulse. In each pulse w T ave we may distinguish 

 a systolic and a diastolic phase; the former, making due allowance 

 for transmission, corresponds with the time during which the aortic 

 valves are open, and blood is streaming from the heart to the aorta, 

 the latter represents the period during which the aortic valves 

 are closed and the arteries are shut off from the heart. In Fig. 214 

 the systolic phase extends from s to d, the diastolic from d to s f . 



Explanation of the Catacrotic Waves. It has been found 

 difficult to give an entirely satisfactory explanation of the catacrotic 

 waves or, to speak more accurately, it is difficult to decide between 

 the different explanations that have been proposed. Concerning 

 the dicrotic wave, it may be said that tracings from different ar- 

 teries show that, like the main pulse wave, it has a centrifugal 

 course, that is, it starts in the aorta and runs peripherally with the 

 same velocity as the main wave upon which it is superposed. More- 

 over, simultaneous tracings of the pressure changes in the heart and 

 in the aorta show that the closure of the semilunar valves is synchro- 

 nous with the small depression or negative wave (d, Fig. 214) which 

 immediately precedes the dicrotic wave. The general belief, there- 

 fore, is that the dicrotic wave results from the closure of the semi- 

 lunar valves. When the distended aorta begins to contract by 

 virtue of the elasticity of its walls, it drives the column of blood in 

 both directions. Owing to the position of the semilunar valves 

 the flow to the ventricle is prevented; but the interposition of this 

 sudden block causes a reflected wave which passes centrifugally 

 over the arterial system. The dicrotic wave is preceded by a 

 small negative wave or notch in the curve which marks the time 

 of closure or just follows the closure of the semilunar valves. The 

 sequence of events as pictured by Mackenzie* is as follows: "As 

 soon as the aortic pressure rises above the ventricular the valves 

 close. At the moment this happens the valves are supported by 

 the hard, contracted ventricular walls. The withdrawal of the 

 support by the sudden relaxation of these walls will tend to produce 

 a negative pressure wave in the arterial system. But this negative 



* Mackenzie, "The Study of the Pulse and the Movements of the Heart." 

 1902. 



