"Nor in the matter of the pulse am I satisfied with the efficient cause thereof which he, following Aristotle, has laid down as the same at the systole as at the diastole, to wit: an effervescence of the blood like that produced in boiling. For the movements aforesaid are sudden strokes and swift beats; while in fermentation or ebullition nothing rises up and collapses thus, as it were in the twinkling of an eye, but there is a slow swelling with a sufficient subsidence. By means of dissection, moreover, one can discern for oneself that the ventricles of the heart are expanded as well as filled by the constriction of the auricles and are increased in size proportionately, according as they are filled more or less; and that the expansion of the heart is a movement of a certain violence, produced by impulsion, not by attraction[227] of some sort."[228] In a letter written four years after the publication of the Exercises to Riolanus and two years after that of the treatise On Generation, Harvey sets forth anew, with admirable clearness and brevity, his doctrine as to the nature and cause of the systole of the ventricles. In this he stands upon purely modern ground as an observer, and his words are free from all Aristotelian tinge. Referring to another physiologist he says:-- "I could wish, however, that he had observed this one thing, namely, that the motion which the heart enjoys is of a threefold kind, to wit: a systole, in which the heart contracts itself and drives out the blood contained in it; and then a certain relaxation, of a character contrary to the foregoing motion, a relaxation in which the fibres of the heart which make for motion are slackened. The two motions aforesaid are inherent in the very substance of the heart, just as in all other muscles. Finally, there takes place a diastole, in which the heart is expanded by blood impelled into its ventricles out of the auricles; and the heart is incited to its own contraction by this filling and expansion of the ventricles; and the motion aforesaid always precedes the systole, which follows at once."[229] Harvey materially clarifies his doctrine of the nature and cause of the heart-beat in the following admirable summary. In the second Exercise to Riolanus he says:--