CHAP, iv.] HEARING. 1339 



envelope. The contour of the bony labyrinth follows in a general 

 way only, not closely the contour of the membranous labyrinth ; 

 hence the perilymph space is not uniform but irregular. In some 

 places, as for instance in the convexities of the semicircular canals, 

 and where the nerves join it, the membranous labyrinth is fixed to 

 the bony envelope, the periosteum of the latter being continuous 

 with the dermis of the former or broken only by small lymph 

 spaces. And where in other situations the perilymph cavity is 

 large, it is much subdivided, in some places more than in others, 

 by bridles and bands of connective tissue. Where the vestibule 

 abuts on the median wall of the tympanum, in the situation of 

 the fenestra ovalis, which is placed over against the line of division 

 of utricle and saccule (Fig. 162) the space contains few such 

 bridles, and here a considerable portion of the perilymph is 

 gathered into what is sometimes spoken of as the ' cisterna.' In 

 the hoops of the semicircular canals the membranous canal, 

 much smaller in section than the bony canal, seems imbedded 

 in the connective tissue of the latter, leaving a considerable space, 

 broken by some few bridles, for the perilymph. In other places 

 the bands of connective tissue, passing from the inner lining of the 

 bony envelope to the walls of the membranous sac, are so abundant 

 that the perilymph space becomes a labyrinth of small irregular 

 passages. Nevertheless, however broken up, the whole perilymph 

 space of the vestibular division of the ear is a continuous space, 

 and the pulses given to it by the movements of the stapes 

 sweep over the whole of it. 



823. The auditory nerve, both the vestibular and the coch- 

 lear division, plunging into the floor of the cranium together with 

 the facial nerve and the nervus intermedius by the canal known 

 as the meatus auditorius internus (Fig. 163 m.i.\ and traversing 

 some compact bone continuous with the compact shell of the bony 

 labyrinth, reaches the labyrinth at the open angle between the 

 base of the cochlea and the vestibule. Here the cochlear nerve 

 passes to the cochlea in a way which we shall presently describe. 

 The vestibular nerve consists of two branches ; one (ramus superior), 

 lying above the other, is distributed to the utricle, and to the 

 superior and horizontal semicircular canals, the other lying beneath 

 the former ends in the saccule and in the posterior semicircular canal. 

 In the utricle the nerve comes into connection, in a manner which 

 we shall study in detail, with an area of modified auditory epithe- 

 lium in the form of an oval low swelling, the macula acustica 

 (Fig. 175 ni.u.), and the connection of the nerve with the saccule is 

 likewise in the form of a macula, (Fig. 175 ra.s.); the maculae of the 

 utricle and of the saccule are the only parts of these two structures 

 in which the epithelium has any connections with the auditory nerve. 

 In the case of each of the semicircular canals the nerve is in connec- 

 tion with a part and a part only of each ampulla. The area of 

 modified auditory epithelium has in each ampulla the form of a 



