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CH. II.] studied the vascular system; and he seems to have perceived that the brain is the seat of sensation. In fine, philosophers, generally, in adopting four causes, have been divided between fire, water, and, as with Diogenes, air; which he held to be the origin of all secondary operations.

Note 13, p. 25. Others, as Critias.] The opinion that blood differs from the other fluids and has an independent vitality, has prevailed, no doubt, in all ages; but Aristotle placed it, as well as its analogue, the ichor, which in molluscs, insects, &c., among insentient and  parts, such as bone, nails, cartilage, and other like parts. It may be added, too, that the brain was so considered. "To conceive," Hunter observes, "that the blood when circulating is endowed with life, is perhaps carrying the imagination as far as it can go, but the difficulty arises from its being fluid, as the mind is not accustomed to the idea of a living fluid. But when all the circumstances of this fluid are considered, the idea that it has life within it, may not appear so difficult to comprehend, for every part is formed, as we grow, out of the blood, and if it has not life previous to this operation, it must acquire it in the act of formation." One of the great proofs of the blood's vitality is to be found in coagulation, as the blood, when circulating, is not subject to certain laws to which it is subject when removed from the vessels.