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60 THE LAW WHICH REGULATES 
the amount of active diastole of the heart, can be altered, by varying 
the diameter of the small arteries of the coronary system, their con- 
traction producing a greater, and their dilatation by facilitating the 
flow of blood through the capillaries a less, degree of diastolic enlarge- 
ment of the ventricular cavities. . 
From the above argument, therefore, the amplified range of pres- 
sure and time depending on change in heart capacity, and the change 
in capacity being caused by modification in the calibre of the smaller 
coronary arteries, it is almost a logical necessity that the function of 
the pneumogastric nerve is to regulate the degree of tonicity of those 
vessels, and Dr. Brown Séquard, from entirely different facts, has also 
published it as his belief that the pneumogastrics contain fibres which 
contract the small coronary vessels.* 
It will be noticed that throughout this paper it has been assumed 
that the systole never recommences until the ventricular cavities are 
completely filled, that is, until a pressure equilibrium has been arrived 
at in the interior of the heart. Perhaps it is the absence of pressure 
which admits of the heart recontracting, but this is a doubtful point, 
and until more is known as to the mechanism of muscular action in 
general, it is probable that the question as to the reason why the heart 
recommences to: beat ata particular moment will remain unsettled. 
Sir J. Paget,f when he pointed out the relation of rhythmic nutrition 
to rhythmic action of nerves and muscles, laid the foundation for a 
scientific treatment of the subject, and the law which it has been my 
endeavour to substantiate, is only a precise method of expressing that 
relation. 
The following summary of the main features in the circulation, as 
they appear to me, may assist in explaining some of the previous 
arguments. 
The circulation of the blood is maintained by the repeated con- 
traction of the heart. Hach cardiac revolution is divided into three 
parts, the systole, the diaspasis, and the diastole. The following laws 
hold with regard to the length of these intervals. 
I. The systole together with the diaspasis, or in other words, the 
first cardiac interval, varies as the square root of the whole revolution. 
IJ. The systole varies as the square root of the diastole. 
III. The diaspasis varies very slightly with different pulse-rates. 
The amount of work that the heart has to perform in maintaining 
the circulation depends on two sets of changes which may occur in 
* See “ Principles of Human Physiology.” By Dr. Carpenter, 1869, p. 219. 
Foot-note. 
+ “Croonian Lecture.” Royal Society, 1857. 
