[a] the filaments (I have already often told you that these come from the innermost part of the brain and compose the marrow of the nerves) are so arranged in every organ of sense that they can very easily be moved by the objects of that sense and that
[b] when they are moved, with however little force, they simultaneously pull the parts of the brain from which they come, and by this means open the entrances to certain pores in the internal surface of the brain; [and that]
[c] the animal spirits in its cavities begin immediately to make their way through these pores into the nerves, and so into the muscles that give rise to movements in this machine quite similar to [the movements] to which we [men] are naturally incited when our senses are similarly impinged upon.
This [in the Figure], if fire A is near foot B, the particles of this fire (which move very quickly, as you know) have force enough to displace the area of skin that they touch; and thus pulling the little thread cc, which you see to be attached there, they simultaneously open the entrance to the pore [or conduit] where this thread terminates [in the brain]: just as, pulling on one end of a cord, one simultaneously rings a bell which hangs at the opposite end. Now the entrance of the pore or small conduit, being thus opened, the animal spirits from cavity F [the ventricle] enter and are carried through it - part into the muscles that serve to withdraw this foot from the fire, part into those that serve to turn the eyes and head to look at it, and part in those that serve to advance the hands and bend the body to protect it. [From Descartes, 1664]