ON SPHYGMOGRAPHY. 73 
heart, rests between its pulsations; for after the rush of blood into 
the arteries immediately the sigmoid valves open, during the rest of 
the systole the blood which leaves the heart is employed in retaining 
the higher pressure in the vessels, as will be explained more fully 
further on. 
We are now ina position to consider the human sphygmograph 
tracing from the wrist, and on looking at Fig. 2 («), which is from a 
slow pulse, it is evident that in all respects it closely resembles that 
taken with the sphygmoscope from the carotid of the horse, which has 
been discussed above, and there is every reason to believe that the 
details originate from similar causes. The primary rise (a) is followed 
by a gentle fall; this is soon broken by the shock-wave (4) consequent 
on the closure of the aortic valve, and is followed by the secondary 
rise (c), which commences when the centrifugal current is augmented 
by the recommencing onward current in the aorta. The sphygmo- 
diastole is remarkably uniform and uninterrupted. 
Between the tracings of slow and quick pulsings there is at first 
sight not much resemblance, but it is not difficult to obtain a series 
between them exhibiting every intermediate condition (Fig. 2, a, 8, 7). 
The most important cause of the variations exhibited by pulses of dif- 
_ ferent rapidities, is that the ratio between the length of the sphygmo- 
~ systole and the sphygmodiastole is not constant. For instance, when 
the pulse is 114 a minute, the sphygmosystole occupies just one half 
the beat; but when it is 40 a minute, it only occupies one quarter of 
the whole revolution. It is evident that this must influence the 
general appearance of the trace, and as the length of the sphygmo- 
systole never varies in health for any given pulse-rate,* a knowledge 
of the ratio of the length of sphygmosystole to that of the whole beat 
is sufficient datum for determining the pulse-rate. In the paper just 
referred to I have given several measurements of these ratios, and have 
shown that the length of the sphygmosystole maintains a very definite 
relation to the length of the beat, varying as its cube root, consequently 
when the length of the revolution increases in the series 1, 8, 27, 64, Page 105. 
&c., the sphygmosystole only does so as 1, 2, 3, 4, &c., so that if we 
call the rapidity of the pulse z, and the number of times that the 
a sphygmosystole is contained in the beat y’, the length of the sphyg- 
mosystole can be found from the equation 
ay’ = 47 3/2. 
Further, the cardio-sphygmographf shows that the interval between 
the closure of the aortic valve and the commencement of the secondary 
* “Proceedings of the Royal Society,” No. 120,1870. (Supra, p. 14.) 
+ “Journal of Anatomy and Physiology,” May, 1871. (Supra, p. 27.) 
Mae 
