528 CIRCULATION OF BLOOD AND LYMPH. 



waves. The most constant of these latter waves occurs usually 

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

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



Fig. 214o. Record of the radial pulse taken with a Frank-Fetter sphygmograph.l After Fetter.) 



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 wave 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'. 



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 



