70 ON SPHYGMOGRAPHY. 
Fig. 2. Sphygmograph tracings, all taken from the same individual in health, 
under different conditions, to show the effect of difference in pulse-rate on the trace. 
a. Pulse-rate 44 in a minute. 
B. ” 63 ” 33 
Y: ” 72 ” ” 
8. y 206, 
é. ” 137 ” ” 
mN 
= ” 172 ” ” 
They are all drawn on one scale and read from left to right. 
the sphygmograph trace of slow pulses the closure of the aortic valve 
occurs when the lever is at the lowest point of the notch that is nearly 
always present in the sphygmosystole, which is also clearly seen in 
the lower of the two tracings in Fig. 1, the curved line 3 cutting it 
exactly at the place which represents the moment when the valve 
shuts. Consequently, in the upper trace, the line 3 must also cut it 
at the same time, and this occurs when the retrograde current is at 
its maximum, as would be expected, the valve being closed by the 
regurgitating blood. But it may be asked, how is it that the back- 
ward current between the lines 3 and 4 in a, Fig. 1, is associated with 
a rise in the sphygmoscope trace? and to explain this clearly it will 
be necessary to refer to some of the elementary principles which 
operate in the transmission of currents through elastic tubes. First, 
it can be demonstrated that increase in the diameter of an artery may 
originate from two quite independent sets of causes, one being the 
simple result of the heart sending more blood into it in a given time 
than it can dispose of, the other being a shock-expansion, comparable 
with the waves of condensation and rarefaction in the air, which con- 
stitute sound. Most of the important elements of the pulse-trace are 
