There are indications that the function of muscle, though unknown to Aristotle, was known not long after his time,[222] and in Galen's time that function was entirely familiar, he styling the muscles "the organs of voluntary movement," and calling their contraction their "systole," a term which has survived only in connection with the heart and arteries.[223] For Harvey, born more than thirteen centuries after Galen's death, the function of muscle was a portion of ancient knowledge; and in his treatise On the Motion of the Heart and Blood, he expressly states that the heart, including the auricles, is muscular both in structure and in function. The opinions of Harvey's day rendered these statements by no means superfluous.[224] Naturally, therefore, in accepting the aid of the Aristotelian seething of the blood in connection with the heart-beat Harvey utilized only the force of expansion thus generated, and obtained from muscle the force of contraction which he required. Indeed, the conception of the auricles and ventricles as muscular force-pumps was fundamental to his doctrine of the circulation. Moreover, we have found Harvey careful to limit and mitigate the expansion of the blood, he saying to Riolanus:-- "I accept Aristotle's parallel with pottage or milk with this proviso, that the rising or falling of the blood is not brought about by vapors, or exhalations, or spirits, excited into some vaporous or aërial form, and is caused, not by an external agent, but by an internal principle, and is regulated by nature."[225] Long before, indeed, he had jotted down a terse statement among his lecture notes which is fatal to any extreme development of the Aristotelian idea. In dealing with the action of the heart he had written:-- "To what end? Aristotle: To none, but a passive process, as in boiling pottage. But when wounded it gives out not wind, but blood."[226] Harvey, therefore, could do no less than criticize adversely his famous contemporary, the philosopher Descartes, for accepting in its entirety Aristotle's doctrine of the heart-beat. Referring to Descartes he says:--