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But as the animal faculty is one, and its proximate vehicle the animal spirits is homo geneous, so the nervous or cerebral substance which conducts these spirits is in its own nature uniform and indifferently competent to either function ; it being dependent upon two accidental circumstances, whether this sub stance conduce to motion, to sensation, or to motion and sensation together. The first circumstance is the degree of hardness or softness ; a nerve being adapted to motion, or to sensation, in proportion as it possesses the former quality or the latter. Nerves extremely soft are exclusively compe tent to sensation. Nerves extremely hard are pre-eminently, but not exclu sively, adapted to motion ; for no nerve is wholly destitute of the feeling of touch. The soft nerves, short and straight in their course, arise from the anterior portion of the encephalos (the Brain proper) ; the hard, more devi ous in direction, spring from the posterior portion of the brain where it joins the spinal chord (Medulla oblongata?) the spinal chord being a contin uation of the After-brain, from which no nerve immediately arises ; the hardest originate from the spinal chord itself, more especially towards its in ferior extremity. prev next |