Definition of "systole"

[…] I did almoſt beleeve, that the motion of the Heart vvas knovvn to God alone: For neither could I rightly diſtinguiſh, vvhich vvay the Diaſtole and Systole came to be, nor vvhen nor vvhere the dilation and conſtriction had its exiſtence.

1653, William Harvey, “The Causes which Mov’d the Author to Write”, in [anonymous], transl., The Anatomical Exercises of Dr. William Harvey […] Concerning the Motion of the Heart and Blood. […], London: […] Francis Leach, for Richard Lownes […], pages 1–2

A double systole catapulted him into full consciousness again, and he promised his uncorrected self that he would limit his daily ration of cigarettes to a couple of heartbeats.

1972, Vladimir Nabokov, Transparent Things, McGraw-Hill, published 1972, pages 78–9

There is no essential virtue in comfort. To be relaxed is good if it is part of a process of systole and diastole. Relaxation comes between phases of tenseness.

1974, Anthony Burgess, The Clockwork Testament