be such a member; in the bloodless tribes, however, it is pro- portional to their state."
Now, if Aristotle understands by the heart that which first appears in the embryo of the chick in ovo, the blood, to wit, with its containing parts the pulsating vesicles and veins, as one and the same organ, I conceive that he has expressed himself most accurately ; for the blood, as it is seen in the egg and the vesicles, is partly similar and partly dissimilar. But if he under- stands the matter otherwise, what is seen in the egg sufficiently refutes him, inasmuch as the substance of the heart, considered independently of the blood the ventricular cone is engen- dered long afterwards, and continues white without any infusion of blood, until the heart has been fashioned into that form of organ by which the blood is distributed through the whole body. Nor indeed does the heart even then present itself with the structure of a similar and simple part, such as might become a primogenial part, but is seen to be fibrous, fleshy, or muscular, and indeed is obviously what Hippocrates styled it, a muscle or instrument of motion. But the blood, as it is first per- ceived, and as it pulsates, included within its vesicle, has as manifestly the constitution which Aristotle held necessary in a principal part. For the blood, whilst it is naturally in the body, has everywhere apparently the same constitution ; when extravasated, however, and deprived of its native heat, imme- diately, like any dissimilar compound, it separates into several parts.
Were the blood destined by nature, however, for the nourish- ment of the body only, it would have a more similar constitution, like the chyle or the albumen of the egg ; or at all events it would be truly one and a single body composed of the parts or juices indicated, like the other humours, such as bile of either kind, and pituita or phlegm, which retain the same form and character without the body, which they showed within their appropriate receptacles ; they undergo no such sudden change as the blood.
Wherefore, the qualities which Aristotle ascribed to a prin- cipal part are found associated in the blood ; which as a na- tural body, existing heterogeneously or dissimilarly, is composed of these juices or parts ; but as it lives and is a very principal animal part, consisting of these juices mingled together, it is
