ON SPHYGMOGRAPHY. 71 
referable to the former of these causes, as is proved by the general simi- 
larity between the two curves « and 8, in Fig. 1. But when there are 
rises in the second, which are quite independent of changes in the 
first, they must evidently be the result of some sudden movement in 
the circulating system, originating a shock, which is not of a character 
to affect the current in any way. It is but reasonable to suppose that 
such should occur as a result of the closure of the semilunar valves, 
and the rise and fall between the vertical lines 3 and 4 in the lower Page 102. 
trace (8) of Fig. 1, commencing immediately on their closure, and 
being unconnected with any current changes, can be nothing else than 
a shock-wave. 
_ At first sight it would seem probable that when the semilunar 
valves had closed, the retrograde current in the aorta would immedi- 
ately cease; but that such is not the case is clearly proved by the 
continuance below the zero line of the current curve for a short period 
after that event (between the lines 3 and 4in the upper trace). It is 
by the combined operation of two causes in the same direction that 
» this result is produced. First, the orifices of the coronary arteries 
being quite close to the semilunar valves, it is evidently necessary 
that the blood which enters them during diastole must be derived 
from the aorta, and so tends to produce in it a retrograde current. 
Secondly, the equilibrium of the arterial system is disturbed during 
the closure of the aortic valve, for immediately the systole has termi- 
nated, the only force tending to prevent the blood from regurgitating 
into the heart is the statical resistance of the ventricular walls, which 
at that moment are closely approximated, causing their cavities to be 
completely obliterated. This resistance is clearly much less consider- 
able than that offered by the heart-walls during the systole, one being 
a statical and the other a dynamical condition; consequently the 
arterial blood rushes back, pushing asunder the ventricular walls, 
and in so doing developing a sufficiently rapid retrograde current to 
close the semilunar valves. The interval thus arising, namely, the 
time between the end of systole and the complete closure of the aortic 
valve, I have called the diaspasis, in a pamphlet on “On the Law 
which Regulates the Frequency of the Pulse,”* and there is consider- 
able evidence to show that when the pulse-rate does not vary it is con- 
stant; also that it varies very slightly with different frequencies of 
heart-action, occupying in slow pulses about 0°002 and in quick ones 
00018 of a minute.t The diaspasis is so short in duration that it has Page 103. 
2 
; 
: 
f 
* Published by H. K. Lewis, 139, Gower Street, London. (Supra, p. 45.) 
t+ In my pamphlet on “ The Law which regulates the Frequency of the Pulse,” 
from a mistake on my own part, a statement is made as to the length of the diaspa- 
sis, which is incorrect. Mathematical friends soon informed me of my error, as, 
