ON THE SPIIYGMOGEAPn, OH PULSE-EECOEDEE. 
133 
the pulmonary arteries — it is not at all necessary to think of 
more than one side of the heart, — that is the left, when we 
attempt to explain the tracings obtained. For as the aorta 
springs from the left ventricle, and the left ventricle receives 
the blood which it pumps onwards, from the left auricle, it is 
the changes which occur during the beat in those two cavities 
which only have any appreciable effect on the pulse at the wrist. 
The movements of the ventricles of the heart, it must also 
be remembered, consist of two separate series of events, one 
being the systole , which is the period during which it is in 
action, contracting and emptying itself of the blood it contains ; 
the other being the diastole , which is the period during which 
it rests and becomes refilled with blood ready to pump out in 
the next systole. This alternation of systole and diastole 
may be compared with the down and up stroke that are made 
in the working of an ordinary pump ; the up-stroke of the 
handle corresponding to the diastole, in which the cylinder 
above the piston is being filled with water ; and the down- 
stroke to the systole, in which the water from the full cylinder 
is being raised, so as to flow from the spout. 
Now to commence with the sphygmograph trace itself. 
The first rush of blood into the arteries at the onset of the 
systolic contraction produces a sudden increase in the diameter 
of the large arteries, which is propagated with considerable 
rapidity along the arterial system to the distant vessels. This 
produces the abrupt and nearly uniform rise (a) which almost 
always forms the most prominent and most considerable feature 
of the sphygmograph trace. This rise is, therefore, the in- 
dication of the commencement of the systole of the left 
ventricle, and it must be borne in mind that it terminates 
before the heart ceases to contract, especially in the larger 
arteries. The primary rise in all properly taken traces is 
continued on as a rounded apex, which is the highest point 
of the trace, and is followed by a fall of considerable rapidity 
as compared with the second descent to be described further 
on. This fall is also generally irregular, especially in slow 
pulses, being broken by a very small wave (6), termed the 4 tidal 
wave,’ by Mr. Mahomed, but perhaps better, the 4 predicrotic 
wave,’ as the latter name does not involve any theory, and will 
be better understood directly. 
The dicrotic rise (c) is the feature in all tracings next most 
important to the primary ascent. It commences at the end 
of the fall just described ; it is less abrupt and shorter than 
the main rise, and is more obtuse or rounded at its summit. 
It is followed by the second fall, which is much less vertical 
than the one which precedes it and more irregular, being 
frequently interrupted by two or three slight undulations. At 
the end of this fall the succeeding primary rise commences. 
