"The primacy of the blood is evident from this also, that the pulse has its origin in the blood. For since a pulsation consists of two parts, to wit: an expansion and a contraction, or a diastole and systole, and since the prior of these movements is the expansion, it is plain that this action is due to the blood, but that the contraction is set a-going in the egg by the pulsating vesicle, as by the heart in the chick, by means of its own fibres as though by an instrument devised for that purpose. It is certain also that the aforesaid vesicle and, at a later time, the cardiac auricle from which pulsation starts, is excited by the blood, which expands to the motion which constricts. The diastole, I say, is produced by the blood which swells up as if with interior spirits; and so Aristotle's opinion as to the heart's pulsation--namely, that it is produced after the manner of ebullition--is in some measure true. For the same thing which we see every day in milk heated over the fire and in the fermentation of our beer, comes into play also in the pulsation of the heart, in which the blood swells as from some fermentation, is expanded, and subsides; and what is brought about in the cases aforesaid by accident and by an external agent, to wit, by adventitious heat from somewhere, is effected in the blood by the internal heat or innate spirits, and is also regulated by the soul in conformity to nature, and is kept up for the health of living things. Pulsation, therefore, is accomplished by a double agency: that is to say, the expansion or dilatation is accomplished by the blood, but the contraction or systole is accomplished in the egg by the membrane of the vesicle, in the fœtus after birth by the auricles and ventricles of the heart; and these alternate and mutually associated efforts once begun, the blood is impelled through the whole body, and thus the life of animals is perpetuated."[207] Nearly two thousand years before Harvey's time Aristotle had said:-- "The volume of leaven[208] changes from small to great, by its more solid part becoming liquefied and its liquid, vaporized.[209] This is brought about in animals by the nature of the psychical heat, but in the case of leaven by the heat of the blended juices."[210] Moreover, Aristotle, as Harvey says, had likened to "ebullition"[211] what Aristotle himself described as "the pulsation which occurs at the heart, at which the heart is always to be seen incessantly at work." "For," says Aristotle, "ebullition takes place when liquid is vaporized[212] by heat; for it rises up owing to its bulk becoming greater."[213] He continues:--