CHAP, iv.] 



HEARING. 



199 



their relations to the perilymph space and as to the manner in 

 which the auditory nerve ends in them. One division is that part 

 of the sac which enters into the construction of the cochlea, and is 

 called the canalis cocMearis (Fig. 176, Coch.). The other division 

 comprises the rest of the sac. The two correspond respectively to 

 the cochlea and the vestibule of the bony labyrinth, including 

 with the vestibule the semicircular canals ; we may speak of _them 

 as the cochlear and vestibular portions of the sac. As we saw In 

 studying the cranial nerves the auditory nerve ( 618), though 

 usually" spoken of as one nerve, really consists of two nerves, 

 different in origin, in ending, and to a certain extent in structure ; 

 one of these two, distributed to the cochlear division of the mem- 

 branous labyrinth, we called the cochlear nerve, the other, dis- 

 tributed to the vestibular division, we called the vestibular nerve. 

 The vestibular division of the membranous labyrinth consists 

 of an oval sac, about 6 mm. long, the utricle (utriculus) (Fig. 176, 

 U), and lying below this a smaller, 3 mm. in diameter, more 

 spherical, though somewhat oval flattened sac, the saccule (sac- 

 culus) (Fig. 176, S). Into the utricle open both ends of each of the 



AS.C 



IfS.C 



FIG. 176. THE MEMBRANOUS LABYRINTH AND THE ENDINGS OP THE AUDITORY 



NERVE. 



The figure is wholly diagrammatic, and is introduced as giving a simpler view of the 

 essential parts *of Fig. 175 ; it should be used only in conjunction with that. 



U. utricle. S. saccule. A.S.C. Superior (or anterior), P.S.C. posterior, H.S.C. 

 horizontal, semicircular canals. 



Coch. The canalis cochlearis represented as a tube partially unrolled, c. canalis 

 reunions, joining the saccule with the canalis cochlearis. a.v. ductus endo- 

 lymphaticus, shewing its origin from both saccule and utricle, and its dilated 

 blind end, the saccus endolymphaticus. 



A.N. The auditory nerve ending in the cristae of the ampullae, in the maculae of 

 the utricle and" saccule, and along the whole length of the canalis cochlearis. 

 The branch of the vestibular division of the nerve ending in the saccule 

 remains in close contact with the cochlear division, longer than does the rest 

 of the vestibular division ending in the utricle and ampullas (the branch to 

 the posterior canal should have been represented as lying in contact with that 

 to the saccule.) 



