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[1]
placed it, as well as its analogue, the ichor, which circu-
lates in molluscs, insects, &c., among insentient and excre-
mentitious parts, such as bone, nails, cartilage, and other
like parts. It may be added, too, that the brain was so
considered. "To conceive," Hunter[2] 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.
