Page 269. 
30 CONSTRUCTION AND USE OF A 
(3.) The first cardio-arterial interval is that which occurs between 
the commencing systolé at the heart and its indication in an artery 
(the radial). 
(4.) The conjugate cardio-arterial interval is that which occurs 
between the commencing systolic rise in on artery and the closure of 
the aortic valve at the heart. 
(5.) The second cardio-arterial interval is that which occurs 
between the closure of the aortic valve at the heart and its indication 
in an artery.* 
On comparing the lengths of the first cardio-arterial interval with 
different rates of pulse, it is found that as the pulse is slower, so this 
interval is longer, and that its length does not increase as rapidly as 
the pulse beat, but as its square root. Consequently if the number of 
times that the first cardio-arterial interval is contained in its component 
pulsation is represented by z and the rapidity of the pulse by x, then 
ke = k./w, and measurements show that the constant quantity & 
equals 39 (or perhaps 39°25) for the sitting posture. A knowledge of 
this equation, therefore, gives a means of calculating the length of the 
first cardio-arterial interval when the rapidity of the pulse is known; 
and as the first cardiac interval also varies as the square root of the 
pulse beat,} it is evident that from its definition the first cardio-arte- 
rial interval must be a constant part of the first cardiac interval, 
whatever the rate, and this has been found to be the case by inde- 
pendent measurement. Another necessary result from these equations 
is, that the conjugate cardio-arterial interval varies inversely as the 
square rovt of the heart’s rapidity. 
The length of the second cardio-arterial interval can also be found 
by subtracting the length of the conjugate cardio-arterial interval 
from that of the first arterial interval, which varies inversely as the 
cube root of the pulse rate,t and by this means it has been found to 
vary very little with different rapidities of pulse, being a little longer 
in the slower pulses. 
In the sphymograph traces of slow pulses the major descent of the 
first arterial interval is broken by a notch, and it is found that the 
* In the above definitions it has been assumed that the sphygmograph trace gives 
indications of the closure of the aortic valves ; and in the measurements to be referred: 
to below, the secondary rise, which puts so abrupt a termination to the major fall in 
each pulsation, is considered to be caused by the closure of these valves, as generally 
assumed ; though Dr. Sanderson has arrived at a different conclusion (“Medical 
Times and Gazette,” March 25, 1871), from evidence which seems to be anything but 
convincing. 
+ “Journal of Anatomy and Physiology,” Nov., 1870. “ On Cardiograph 
Traces from the human chest-wall.” (Supra, p. 18.) 
t “ Proceedings of the Royal Society,” No. 120, 1870. “On the relative duration 
of the component parts of the radial sphygmograph tracein health.” (Supra, p. 14.) 
