tion of the blood,) urging the blood in a ceaseless round through every part of the body, we see that the blood must exist before the heart, both in the order of generation and of nature and essence. For the blood uses the heart as an instrument, and moreover, when engendered it continues to nourish the organ by means of the coronary arteries, distributing heat, spirits, and life to it through their ramifications.
We shall have further occasion to show from an entire series of anatomical observations, how this rule of Aristotle in respect of the true priority of the parts is borne out. Meantime we shall see how he himself succeeds in duly inferring the causes of priority in conformity with his rule.
" After the prime part viz. the heart is engendered/' he says, " the internal parts are produced before the external ones, the superior before the inferior; for the lower parts exist for the sake of the superior, and that they may serve as instruments, after the manner of the seeds of vegetables, which produce roots sooner than branches."
Nature, however, follows no such order in generation; nor is the instance quoted invariably applicable ; for in beans, peas, and other leguminous seeds, in acorns, also, and in grain, it is easy to see that the stem shoots upwards and the root down- wards from the same germ ; and onions and other bulbous plants send off stalks before they strike root.
He then subjoins another cause of this order, viz. : " That as nature does nothing in vain or superfluously, it follows that she makes nothing either sooner or later than the use she has for it requires." That is to say, those parts are first engen- dered whose use or function is first required ; and some are begun at an earlier period because a longer time is requisite to bring them to perfection ; and that so they may be in the same state of forwardness at birth as those that are more rapidly produced. Just as the cook, having to dress certain articles for supper, which by reason of their hardness are done with difficulty, or require gentle boiling for a great length of time, these he puts on the first, and only turns subsequently to those that are prepared more quickly and with less expenditure of heat ; and further, as he makes ready the articles that are to come on in the first course first of all, and those that are to be presented in the second course afterwards ; so also does nature
