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blood at a certain definite pressure and for a certain definite 
time 5 the ventricles can likewise only develop a certain pres- 
sure and maintain it till a certain definite amount of work has 
been executed* Therefore the ventricles immediately they 
commence to contract exert their full systolic pressure, and 
maintain that pressure until they are empty. But it is evident 
that the time necessary to empty them of a definite amount 
of blood under these conditions, must depend on the rapidity 
of the flow of blood through the small arteries and capillaries ; 
when the flow is halved the systolic time is doubled, if no other 
force come into 'play ; in other words, the length of the systole 
is dependent on the arterial resistance. If therefore the length 
of the diastole did not affect the length of the systole, when 
the former is doubled the latter would be doubled also ; but 
from the law that the systole varies as the cube root of the 
pulse-length, it may be inferred that such is not’ the case; and 
from a study of tracings from the movements of the heart, as 
they affect the chest-wall — termed cardiograph tracings — many 
reasons may be given in favour of the law that the nutrition 
of the heart’s walls varies as the square root of the diastolic 
period,* during which only the blood can traverse the capillaries 
of the coronary system of vessels, which supply the ventricular 
walls. 
There is yet more to be learnt from the sphygmograph trace. 
As previously stated, the length of the systole does not vary 
for any given pulse-rate ; but the blood-pressure in the arterial 
system varies independently of the pulse-rate altogether ; con- 
sequently, the length of the systole does not depend on the 
blood-pressure. This result, somewhat paradoxical at first 
sight, can only be explained on one assumption, which is that the 
variations in the arterial blood-pressure must affect the nutri- 
tion of the heart in such a way that the extra work to be 
performed, when it is raised, produces a corresponding extra 
power in the ventricle to overcome the added obstacle to its 
contraction. The coronary vessels being direct offshoots from 
the aorta, their blood must vary in pressure with that in their 
main stem ; and this, in addition to the above-stated facts, all 
tends to prove that the nutrition of the walls of the heart varies 
directly as the bloocl-pressure in the arterial system. 
These peculiarities in the mechanism of the cardiac action 
have a definiteness about them which can be obtained only 
through the employment of the sphygmograph ; and I hope 
that the above remarks will lead others to repeat my measure- 
ments, in doing which I am satisfied that they will substantiate 
the accuracy of my results. 
* See The Law which regulates the Frequency of the Pulse , 11 Journal of 
Anatomy and Physiology.” May and November, 1873. 
