io6 THE CIRCULATION OF THE BLOOD AND LYMPH 



When the semilunar valve becomes incompetent in disease, or is 

 rendered insufficient in animals by the artificial rupture of one or 

 more of its segments, the dicrotic wave, as will be readily understood 

 from the manner in which it is produced, either disappears altogether 

 or is markedly enfeebled. But apart from any anatomical lesion or 

 functional defect in the aortic valves, the prominence of the wave 

 varies with a great number of circumstances, some of which are in a 

 measure understood, while others remain obscure. It varies in par- 

 ticular with the abruptness of discharge of the ventricle and the ex- 

 tensibility of the arteries. The conditions are usually favourable when 

 the arterial pressure is low, for the blood then passes quickly from the 

 ventricle into the arteries, which, already only moderately tense, are 

 easily dilated by the primary wave, then sharply collapse, and are again 

 abruptly distended when the dicrotic wave arrives. And, in fact, an 

 exaggeration of the dicrotic wavelet may be artificially produced by 

 nitrite of amyl (Fig. 102, p. 209), a drug which lessens the blood-pressure 

 by dilating the small arteries. Muscular exercise (Fig. 101, p. 209), 

 running or bicycling, for instance, has a similar effect on the sphygmo- 

 gram, although the explanation can scarcely be the same, since the blood- 

 pressure mounts rapidly when moderate exercise begins, and only 

 gradually falls during its continuance, with an abrupt decline to normal 

 or below it on cessation of work (Bowen). The increase in the pulse- 

 rate may have something to do in this case with the exaggeration of the 

 dicrotism, which is very frequently, although by no means invariably, 

 associated with a rapidly -beating heart, and therefore is often seen in 

 fever. On the other hand, in certain diseases associated with a high 

 arterial pressure, the dicrotic elevation almost disappears. Ather- 

 omatous arteries, being very inextensible, do not allow a dicrotic pulse. 



Since the pulse represents a periodical increase and diminution in 

 the amount of distension of an artery at any point, the line joining 

 all the minima of the pulse-curve will vary in its height above the 

 base-line, or line of no pressure, according to the amount of permanent 

 distension, i.e., permanent blood-pressure, which the heart in any given 

 circumstances is able to maintain. Any circumstance that tends to 

 lessen the permanent distension will cause a fall of the line of minima, 

 and any circumstance tending to increase the distension will cause that 

 line to rise. If, for example, the arm be raised while a pulse-tracing 

 is being taken from the wrist, the line of minima falls because the 

 permanent pressure in the radial artery is diminished. 



The form of the pulse-curve varies in the different arteries, and 

 therefore in making comparisons the same artery should be used. 

 When the wave of blood only enters an artery slowly, the ascending 

 part of the curve will be oblique. This is normally the case in a 

 pulse-curve of a distant artery, such as the posterior tibial. The 

 height of the wave is also less than in an artery nearer the heart, such 

 as the carotid, or even the axillary, where the primary elevation is 

 relatively abrupt (Fig. 39, p. 104). 



Anacrotic Pulse. As a rule, the ascent of the tracing is unbroken 

 by secondary waves, but in certain circumstances these may appear 

 on it. This condition, which, when well marked at any rate, may 

 be considered pathological, is called anacrotism (Fig. 38). It is seen 

 when the discharge of the left ventricle into the aorta is slow and 

 difficult e.g., in cases where the orifice of the aorta has been 



