9. While resting, then, the whole weight of the body on this substance of the bones, as on some columns that carry a mass of building, she did not implant the bone undivided through the whole structure: for in that case man would have remained without motion or activity, if he had been so constructed, just like a tree that stands on one spot without either the alternate motion of legs to advance its motion or the service of hands to minister to the conveniences of life: but now we see that she contrived that the instrument should be rendered capable of walking and working by this device, after she had implanted in the body, by the determining spirit which extends through the nerves, the impulse and power for motion. And hence is produced the service of the hands, so varied and multiform, and answering to every thought. Hence are produced, as though by some mechanical contrivance, the turnings of the neck, and the bending and raising of the head, and the action of the chin, and the separation of the eyelids, that takes place with a thought, and the movements of the other joints, by the tightening or relaxation of certain nerves. And the power that extends through these exhibits a sort of independent impulse, working with the spirit of its will by a sort of natural management, in each particular part; but the root of all, and the principle of the motions of the nerves, is found in the nervous tissue that surrounds the brain.