ON GENERATION. 387 



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 



