528 CIRCULATION OF BLOOD AND LYMPH. 



or low by estimating the force with which the wave presses upon 

 the fingers, or the pressure necessary to occlude the artery. A 

 similar inference may be drawn from the character of the sphyg- 

 mogram, and especially from the relative size and position of the 

 dicrotic wave. When this latter wave falls at or near the base line 

 of the curve it indicates a low arterial pressure, since under these 

 circumstances the artery collapses readily after its first systolic 

 expansion (see Fig. 216). Since the introduction of the sphyg- 

 momanometer (p. 501), however, it seems evident that this instru- 

 ment must be appealed to whenever the determination of blood- 

 pressure is a matter of importance. 



Venous Pulse. Under usual conditions the pulse wave is lost 

 before entering the capillary regions, but as a result of dilatation in 

 the arteries of an organ the pulse may carry through and appear in 

 the veins, in which it may be shown, for instance, by the rhythmical 

 flow of blood from an opened vein. The term venous pulse, how- 

 ever, as generally used applies to an entirely different phenomenon, 

 namely, to a pulse observed especially in the large veins (jugular) 

 near the heart. The pulse in this case is not due to a pressure wave 

 transmitted . through the capillaries, but to pressure changes of 

 both a positive and negative character occurring in the heart and 

 transmitted backward into the veins. The venous pulse that has 

 this origin may usually be seen and recorded in the external (or 

 internal) jugular. Under pathological conditions, especially when 

 the flow through the right heart is more or less impeded, it may 

 be plainly apparent at a further distance from the heart and may 

 cause a noticeable pulsation of the liver, which is designated as a 

 liver pulse. The venous pulse curve has been much studied in 

 recent years.* It is somewhat complicated and an explanation 

 of some of its details has not been agreed upon, but there can be no 

 doubt that when properly interpreted it will throw much light upon 

 the pressure changes in the heart, and will afford a valuable means 

 of diagnosis in cases of valvular lesions and other pathological 

 conditions of the heart. It is evident also that the venous pulse 

 gives a ready means of determining the rate of beat of the auricles, 

 just as the arterial pulse enables us to count the beats of the ven- 

 tricles, and in this way records of the venous pulse are important 

 in the interpretation of irregularities in the beat of the heart 

 (arrhythmia). 



As usually recorded the venous pulse shows three positive 

 waves, designated commonly as the a, c, and v waves, and three 

 negative waves. Of the three positive waves, the a wave marks, 



*See Mackenzie, " The Study of the Pulse," 1902; also Lewis, in Hill's 

 " Further Advances in Physiology," New York, 1909. 



