THE NERVOUS SYSTEM 



201 



one below the scala media (Fig. 130), the two channels being 

 connected with each other only at the distal end of the scala 

 media. The upper channel is the scala vestibuli and the lower 

 one the scala tympani. The terms "upper" and " lower" are 

 used with reference to the cochlea, the apex of the spiral being 

 "above" and the base "below." The bony labyrinth is pierced 



Fig. 130. — A, diagram of mammalian ear; 

 B, cross section of one of coils of cochlea. 

 an, auditory nerve; cn, cochlear nerve; d, 

 perilymphatic duct; e, Eustachian tube; em, 

 external meatus; g, spiral ganglion; i, incus; 

 M, scala media; ma, malleus; p, pharynx; r, 

 foramen rotunda; s, sacculus; st, stapes; t, 

 scala tympani; tc, tympanic cavity; tm, tym- 

 panic membrane; u, utriculus; v, scala vesti- 

 buli; vn, vestibular nerve. (Modified from 

 Kingsley, Comparative Anatomy of Vertebrates. 

 P. Blakiston's Son and Company. By permission.) 



by two openings toward the middle ear: (1) the fenestra rotunda, 

 located at the base of the cochlea where the scala tympani 

 terminates, and closed by a membrane; and (2) the fenestra 

 ovalis, in the region (vestibulum) of the sacculus, and closed by a 

 bone of the middle ear, the stapes (Fig. 130A). The vestibulum 

 is an enlarged perilymphatic space about the sacculus. The only 

 passageway from the vestibulum to the fenestra rotunda is 



