CIRCULATION OF THE BLOOD. 83 
marked influence on the amount of blood which enters the capillaries 
of the walls of the heart, and consequently infiuences the amount of 
work which the muscular fibres of the ventricle have to perform in 
emptying their interstitial vessels before they can commence con- 
tracting on the blood in their contained cavity. Experiment shows 
that the rapidity of the pulse does not depend on the pressure of the 
blood in the arterial system ;* consequently the length of the syspasis 
is not influenced by the arterial blood-pressure, which is the same 
thing as saying that the force of the cardiac contraction varies directly 
as the blood-pressure; for then the muscular power of the ventricular 
walls to overcome the intramural distension, varying with it, prevents 
its duration from being modified. 
It has been my endeavour to show elsewheret that the force of 
the heart’s contraction is modified by the length of diastole, varying 
as its square root. Such being the case, it is evident that the length 
of the syspasis must vary with that of the diastole, though not tothe 
extent that is found to occur. But the diastolic period being always 
so short, it is evident that the longer it is the more thoroughly does 
the heart-tissue get permeated with blood, in a way which can have 
little or no influence on its nutritive power, but a great effect in 
modifying the length of the syspasis in the direction which is found 
to occur. 
Again, referring to the results of the cardio-sphygmograph obser- 
vations published by me in the “ Proceedings” of this Society (vol. 
xix, p. 318), that paper contains a table of the length of the different 
cardio-arterial intervals; and if from the first cardio-arterial interval, 
as there defined, the length of the syspasis be subtracted at the cor- 
responding rates, it will be found that the remainder of the interval is 
of exactly the same length as the second cardio-arterial interval, 
which, on the assumptions made, it could only be, as both the systole 
and the shock of the closure of the aortic valve are propagated along 
the arteries from the same point under similar circumstances. The 
following table gives the lengths of the first cardio-arterial interval 
from which that of the syspasis as above determined has been sub- 
tracted, and by their side the lengths of the second cardio-arterial 
interval, as copied from the table in the communication referred to; 
their similarity cannot be the result of simple coincidence, as they are 
derived from independent sets of measurements. 
* “Journal of Anatomy and Physiology,” November, 1873. (Supra, p. 51.) 
+ Ibid. vol. viii. (Supra, p. 59.) I (Supra, p. 32). 
