206 ESSENTIALS OF PHYSIOLOGY. 



are due to slight oscillations of the stretched arterial walls, which are 

 magnified and distorted by vibrations set up in the sphygmograph. The 

 portion of the wave from h to c is normally descending, and the pulse 

 is called katacrotic when the primary wave continues to rise almost 

 until the dicrotic notch is reached, the pulse is said to be anacrotic. 



The primary wave is caused by the sudden expansion of the artery 

 during the cardiac systole, the form of the wave depending upon the 

 peripheral resistance. After the first abrupt rise of arterial pressure 

 the blood usually escapes through the peripheral resistance more 

 rapidly than it enters the arterial system from the heart, and the pulse is 

 katacrotic. When the peripheral resistance is very high, e.g. in old age 

 or in Bright's disease, blood continues to enter the aorta during systole 

 more rapidly than it passes through the arterioles ; the arterial pressure 

 continues to rise almost to the end of systole, and the pulse is anacrotic. 



In the aorta the primary wave begins coincidently with the opening 

 of the semilunar valves and the escape of blood into the aorta, as may 

 be seen in fig. 75, which gives a simultaneous record of the endb- 

 cardiac pressure and of a pulse tracing. Since this wave of expansion 

 travels from the aorta to the peripheral vessels, the percussion wave 

 begins in the smaller vessels an appreciable time later than in the 

 aorta. This can be readily observed by taking simultaneously two 

 pulse tracings, one from the carotid artery and one from the radial 

 artery at the wrist. By measuring the interval with the aid of a time 

 marker and noting the difference in distance from the heart of the points 

 at which the tracings are made, the velocity of the pulse wave can be 

 measured. Thus, if the interval is one-tenth of a second and the differ- 

 ence in distance is 0*6 metre, the velocity is 6 metres per second. The 

 length of the wave is from 5 to 6 metres. 



Fig. 75 also shows that the dicrotic wave occurs immediately after 

 the closure of the semilunar valves, and as no corresponding wave is 

 present in the endocardiac pressure tracing, the dicrotic wave must 

 have its origin in the arterial system. It has been thought that as 

 the primary wave passed along the arteries it was reflected, as a 

 secondary wave, from the obstructions set up wherever the arteries 

 branched, and particularly from the arterioles where the branchings are 

 very numerous. This small reflected wave, starting at the periphery, 

 was believed to travel back towards the heart as the dicrotic wave. 

 Such a wave is actually produced in an artificial scheme of the circula- 

 tion where the pulse wave beats upon a peripheral resistance. 



If this view were correct, the distance between the primary and 

 dicrotic waves would naturally be less in the peripheral arteries near the 

 seat of origin of the reflected wave than in the aorta. But the interval 



