on the sensations of the body

"To understand, next, how external objects that strike the sense organs can incite [the machine] to move its members in a thousand different ways: think that [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 be very easily 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 this 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 muscles that give rise to movements in this machine quite similar to [the movements] to which we are naturally incited when our senses are similarly impinged upon.