"In the heart the swelling up from heat of the liquid which is always arriving from the food produces pulsation, for the swelling rises against the outer tunic[214] of the heart; and this process is always and incessantly going on, for the liquid is always and incessantly flowing in, out of which the nature of the blood arises; for the blood is first worked up in the heart. The thing is plain in generation from the beginning; for before the vessels have been marked out the heart is to be seen containing blood. Hence, too, it pulsates more in the young than in the old; for the vapor[215] arises more abundantly in the young. "All the vessels also pulsate and do so simultaneously one with another, because they are dependent upon the heart.[216] This is always moving, so that they, too, are always moving, and simultaneously one with another, when[217] the heart moves. Leaping [of the heart],[218] then, is the reaction which takes place against the condensation produced by cold, and pulsation is the vaporization[219] of heated liquid."[220] In another treatise Aristotle says: "In all animals the blood pulsates in the vessels everywhere at the same time."[221] It is interesting, in a negative way, that his sweeping and faulty references to the pulsation of the vessels put into words no physiological idea except the vague one of "dependence" on the heart. One may be tempted to see in the seething of the heart's blood the source of some of those spirits within the body elsewhere than in and about the heart, of which one gets brief ill-defined glimpses here and there in the genuine works of Aristotle. But no words of his can be adduced to confirm such a conjecture. Evidently, however, the seething of the nascent blood suffices, in Aristotle's eyes, to explain both the phases of the heart-beat; for both the rising and the falling of the wall of the hot central laboratory of the blood are movements as passive apparently as those of the lid of a boiling pot. One may be excused for wondering at the crudity of such a conception; nor is one's wonder lessened by recalling that elsewhere in Aristotle's works he places at the heart the central origin of the bodily movements. But when it is recalled, as well, that Aristotle was totally ignorant of the function of muscle and, therefore, even of the mode of working of the limbs, his doctrine of the heart-beat may seem less amazing.