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BUt what here is most considerable is, that all the most lively, and subtle parts of the blood, that heat hath rarified in the heart, continually enter in abundandance into the cavities of the braine, and the reason why they
go thither rather than any where else, is, because all the blood that issues out of the heart by the great artery bends its course in a direct line thi∣ther ward, and it not being possible for all to get in, because there are none but very narrow pas∣sages, those parts thereof that are the most agi∣tated, and subtlest, only get in, while the rest is dispersed into all the other parts of the body. Now these very subtle parts of the blood make the animall spirits; and they need not, to this end, undergoe any other change in the brain, but only be separated from the other lesse subtle parts of the blood; for what I here call spirits, are but bodyes, and have no other property, un∣lesse tha they are bodies exceeding small, which move very nimbly, as the parts of a flame issuing from a torch: so that they stay not in any one place, but still as some get into the cavities of the brain, some others get out through the pores in the substance of it; which pores convey them into the nerves, and from thence into the muscles, by means whereof they mould the body into all the severall postures it can move.