CH. LIU.] THE AUDITORY NERVE. 



two scalce, and encloses a spiral canal, previously mentioned, 

 called the canalis cochleae (fig. 543). The fluid in the scalce of 

 the cochlea is continuous with the perilymph in the vestibule and 

 semicircular canals. The vestibular portion of the membranous 

 labyrinth comprises two communicating cavities, of which the 

 larger and upper is named the utricle ; the lower, the saccule. They 

 are lodged in depressions in the bony labyrinth, termed respectively 

 fovea hemielliptica and/bvea hemispherica. The membranous semi- 

 circular canals open into the utricle : the canal of the cochlea 

 opens by the canalis reuniens into the saccule. The accompanying 

 diagram (fig. 543) gives the relationship of all these parts to one 

 another. 



Auditory Nerve. All the organs now described are provided 

 for the appropriate exposure of the filaments of the auditory 

 nerve to vibrations. It enters the bony canal (the meatus 

 auditoritis intemus), with the facial nerve and the nervus inter- 

 medius, and, traversing the bone, enters the labyrinth at the 

 angle between the base of the cochlea and the vestibule, in two 

 divisions ; one for the vestibule and semicircular canals, and the 

 other for the cochlea. 



There are two branches for the vestibule, one, superior, dis- 

 tributed to the utricle and to the superior and horizontal semi- 

 circular canals, and the other, inferior, which arises from the 

 cochlear nerve, ends in the saccule and posterior semicircular 

 canal. There can, however, be little doubt that the inferior 

 nerve, although it is contained for some distance in the sheath of 

 the cochlear nerve, is really composed of vestibular fibres. 

 Where the nerve comes in connection with the utricle and 

 saccule, the structure of the membrane is modified and the 

 places are called maculce acousticce. At the ampullae of the semi- 

 circular canals, too, the structure is altered, becoming elevated into 

 a ridge, which projects into the interior of the cavity, forming the 

 crista acoustica. The distribution of the true cochlear nerve 

 occurs along the whole length of the canal of the cochlea. 



The structure of the membranous canals has been given in 

 Chapter XLIX., so we can pass at once to the cochlea. 



This is best seen in vertical section ; the cavity is divided into 

 two scalte, partly by tone (the spiral lamina), partly by mem- 

 brane (the basilar membrane) ; the other end of the basilar 

 membrane is attached to the bone by a ligament (the spiral 

 li'inineni), formerly supposed to be a muscle (Bowman's muscle) ; 

 the two spiral staircases or scalee are named scala vestibuli and 

 scala tympani (fig. 544). At the apex of the cochlea, the spiral 

 lamina ends in a small hamulus, the inner and concave part of 



