viii BLOOD-STEEAM: MOVEMENT IN VESSELS 269 



semilunar valves (point I of the cartographic curve). Since 

 the points &, c, d, e, f of the cardiogram correspond to the same 

 number of points on the sphygmogram at V, c, d', e',f, but all 

 with a delay, equal approximately to the interval aa', it follows 

 that this interval expresses the time occupied by the transmission 

 of the primary wave from the arterial orifice to the carotid, and 

 that the intervals ab, a'b f represent the time of latent systole. 



The interval &c, which can hardly be seen on the cardiogram 

 as a slight drop in the curve, corresponds to the interval Vc' 

 marked by the sharply ascending curve of the primary wave. 

 The interval cd, indicated by a slight rise of the cardiographic 

 line, corresponds to the interval c'd', indicated on the sphygmogram 

 by a tract that is almost horizontal (the plateau}. The interval 

 de, corresponding to d'e', both on the cardiogram and on the 

 sphygmogram shows a slowly descending line : and the intervals 

 ef and e'f correspond in both curves with rapidly descending lines. 



Leaving aside the features c and d which are less conspicuous, 

 and have only a dubious significance, it is evident, or at any rate 

 extremely probable, that the entire interval be represents the 

 period of systolic outflow or evacuation, during which the heart, 

 in consequence of the diminution in its volume, exerts a constantly 

 decreasing pressure on the intercostal space to which the cardio- 

 graph is applied. In the sphygmographic curve these periods are 

 represented by the interval Vc' in which the primary pulse wave 

 is traversing the artery. 



The small interval ef coincides with the beginning of active 

 diastole, during which, owing to the diminished tension of the 

 cardiac muscle, the intercostal space on which the button of the 

 cardiograph rests, sinks in. This corresponds to the lines e'f of 

 the sphygmogram, which represent a true negative wave that pre- 

 cedes the secondary or dicrotic wave. 



To understand the origin of this dicrotic wave it is enough 

 to consider that at the commencement of diastole an enormous 

 difference between aortic and intraventricular pressure arises : this 

 causes the blood column to gravitate towards the semilunar valves 

 which are already closed, throwing them into vibration so that 

 they develop the second sound, and set up the negative wave in 

 the artery, owing to their distension towards the conus arteriosus. 

 The sharp tension of the valves is succeeded by their elastic 

 reaction, which produces the positive dicrotic wave that follows 

 immediately on the negative wave. The great majority of physi- 

 ologists, including Grashey, Edgren, Hoorweg, Hiirthle, are 

 unanimous in accepting this explanation of the dicrotic wave. To 

 us its central origin appears conclusive, on account of the negative 

 wave that precedes it and can only be caused by the rapid 

 recession of the column of blood in the aortic bulb, which 

 distends the valves and pushes them down towards the conus 



