CH. LVI.] 



THE COCHLEA 



791 



an opening filled by membrane, in which is inserted the base of the 

 stapes ; in its posterior and superior walls, five openings by which 

 the semicircular canals communicate with it: in its anterior wall, 



FIG. 491a. Right bony labyrinth, viewed 

 from the outer side. The specimen 

 here represented is prepared by sepa- 

 rating piecemeal the looser substance 

 of the petrous bone from the dense 

 walls which immediately enclose the 

 labyrinth. 1, The vestibule; 2, fen- 

 estra ovalis; 3, superior semicircular 

 canal ; 4, horizontal or external canal ; 

 5, posterior canal; *, ampullae of the 

 semicircular canals; 5, first turn of 

 the cochlea ; 7, second turn ; 8, apex ; 

 9, fenestra rotunda. The smaller figure 

 in outline below shows the natural size. 



2i 



-~ (Summering.) 



FIG. 491&. View of the interior of the left 

 labyrinth. The bony wall of the laby- 

 rinth is removed superiorly and exter- 

 nally. 1, Fovea hemielliptica ; 2, fovea 

 hemispherica ; 3, common opening of 

 the superior and posterior semicircular 

 canals; 4, opening of the aqueduct of 

 the vestibule; 5, the superior; 6, the 

 posterior, and 7, the external semicir- 

 cular canals ; 8, spiral tube of the 

 cochlea (scala tympani); 9, opening of 

 the aqueduct of the cochlea; 10, placed 

 on the lamina spiralis in the scala ves- 



an opening leading into the cochlea. The semicircular canals are 

 described in Chapter LI. 



The Membranous Labyrinth corresponds in general form with 

 the osseous labyrinth. The vestibule contains two membranous 

 sacs, named the utricle and the saccule (fig. 492) ; the utricle com- 

 municates with the three membranous semicircular canals; the 

 saccule communicates with the utricle and with the canal of the 

 cochlea. The vestibular division of the auditory nerve is distributed 

 to the five spots shown in the diagram, namely, the maculae of utricle 

 and saccule, and the cristae of the semicircular canals. The cochlear 

 division of the auditory nerve is distributed to the whole length of 

 the canal of the cochlea. 



The Cochlea. This is shaped like a snail's shell. It is traversed 

 by a central column or modiolus, around which a spiral canal winds 

 with two and a half turns from base to apex. It is seen in 

 vertical section (fig. 493) that this canal is divided partly by bone 

 (the spiral lamina), partly by membrane (the basilar membrane), 



