24. Now this force is that which is considered as residing in the cerebral membranes and the brain, from which it comes that every movement of a joint, every contraction of the muscles, every spontaneous influence that is exerted upon the individual members, renders our earthen statue active and mobile as though by some mechanism. For the most pure form of heat and the most subtle form of liquid, being united by their respective forces through a process of mixture and combination, nourish and sustain by their moisture the brain, and hence in turn, being rarefied to the most pure condition, the exhalation that proceeds from that organ anoints the membrane which encloses the brain, which, reaching from above downwards like a pipe, extending through the successive vertebræ, is (itself and the marrow which is contained in it) conterminous with the base of the spine, itself giving like a charioteer the impulse and power to all the meeting-points of bones and joints, and to the branches of the muscles, for the motion or rest of the particular parts.