AUDITORY NERVE. By anatomists. the A. N. is associated with the facial, and is the seventh in order of origin front the brain. counting from before backwards. The seventh pair consists of the portio Jura or facial, the portio mollis or auditory. and a small intermediate portion. The portio mak apparently commences by some white streaks in the floor of the fourth ventricle; it then runs forward to the back of the petrous portion of the tempond bone. and enters the internal auditory meatus. The facial then leaves it to pass along the canal called the aqueductns fallopii. and the auditory divides into two portions, which diverge—the smaller one posterior for the semieirenlar canals and the vestibule, the other for the cochlea. Those entering the saileircular canals divide into five branches, forming at last a nervous expansion somewhat analogous to the retina.
Several theories have been held at different periods with regard to the manner in Nvhich the nerves terminate in the cochlea, and how sound is transmitted from the latter to the brain. The latest, and that which is at present entertained by most physiologists, is that of M. Schultze. It has been shown, by actual experiment, that when a nerve in connection with a muscle is acted upon by a succession of very rapid strokes from the little hammer of a tetanmotor, and, when the strokes have arrived :it a certain number the second, a stimulus is sent along the nerve exciting the muscle to action. It is in
jthe same Way that M. Schultze supposes the impression of sound to be_ propagated to 'the Nerves of the cochlea, by means of a series of little tetaninotors called the teeth of Corti. who discovered them. They are situated in the spiral lamina, which separates the spiral canal in the interior of the cochlea into an upper and a lower half or soda. The spiral lamina consists of an osseous septum, next to the eeutral axis of the cochlea, and of a membranous layer, which prolongs the osseous septum to the outer wall of the cochlea, thus completing the spiral lamina. This membranous septum is double, and between its layers there is a chamber which contains the teeth of Corti, ranged side by side throughout the whole length of the spiral lamiun, and getting, shorter from base to apex, like the strings of a harp or piano forte. The chamber is tilled up by a tremulous jelly-like fluid. The diagram, fig. 3, represents a perpendicular section of the spiral lamina. When the waves or vibrations of sound strike against the bones of the head, those bones are caused to vibrate; this vibration is transmitted through the head to the bones of the cochlea, which in turn set in motion the tremulous jelly which fills up the membranous chamber, c.