474 CIRCULATION OF BLOOD AND LYMPH. 



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 emphasized and may be detected by the finger. A pulse 

 of this kind is known as a dicrotic pulse. 



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 which immediately 

 'precedes the dicrotic wave. The general belief, therefore, is that the 

 dicrotic wave results from the closure of the semilunar 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 smaller waves, such as the predicrotic, and the postdicrotic, 

 have been explained (Landois)* simply as elasticity waves, that 

 is, as elastic vibrations of the arterial walls. According to other 

 authors, f an important perhaps the chief factor in the produc- 

 tion of the secondary waves is the reflection that occurs from the 

 periphery. Where each arterial stem breaks up into its smallest 

 vessels the main pulse wave suffers a reflection, a wave running 

 backward toward the heart. It is probable that such reflected 

 waves from different areas for instance, from the coronary system, 

 the subclavian system, the mesenteric system, etc. meet in 

 the aorta and may in part summate to larger waves which 

 again pass peripherally. The catacrotic waves, according to 

 this view, probably differ in character in the different arteries, 

 and tracings indicate that this is the case. The radial pulse differs, 

 for instance, from the carotid pulse in the character of its waves. 

 Between these opposite views it is not possible to decide, but it is 

 perhaps permissible to believe that while the dicrotic wave is due 

 primarily to the impulse following upon the closure of the semilunar 

 valves, nevertheless the actual form of this and the other secondary 



* Landois, " Die Lehre von Arterienpuls," 1872. 

 t See von Frey, loc. cit. 



