THE COCHLEA. 



397 



tubes join together at one end, the three communicate with the vesti- 

 bule, by five openings. 



The cochlea, Figs. 74, 77, c, is a little spiral canal, with bony walls, 

 resembling a small snail's shell, whence its name ; it is placed in front 



Fig. 77. 



Fig. 77. Plan of the labyrinth or internal ear, showing its cavity laid open ; enlarged. , the auditory 

 nerve, entering the labyrinth from the cavity of the cranium, s. the bony semicircular canals laid open, 

 showing the membranous canals within them, and their ampullae or enlargements; also the membranous 

 vestibule and saccule, lying in the central portion of the osseous labyrinth named the vestibule. Leading 

 from this is the spiral coil of the cochlea, c, also laid open, so as to show the striated surface of its lamina 

 spiralis. 



The lower figure shows a section through the cochlea, from base to apex, a, is the bony wall of the 

 spiral tube, and ft, the spiral partition, which divides each spire into two canals, named the scalffi, the 

 upper one here being, throughout, the scala tympani, and the lower one, the scala vestibuli. (Arnold.) 



of the vestibule, forming the anterior part of the labyrinth, and meas- 

 ures from base to apex about Jth of an inch. It possesses a bony 

 axis, called the modiolus, formed, as it were, by the coalescence of its 

 spiral turns. A thin spiral bony plate, the lamina spiralis, projects 

 from the sides of the modiolus; this plate, which turns round the 

 modiolus, like the thread of a gimlet, gives attachment, in the recent 

 state, to a double membrane, which contains some remarkable struc- 

 tures, to be presently described, and which is extended across to the 

 outer wall of the spiral canal, Fig. 77, a. The partly bony and partly 

 membranous spiral plate, b, which gradually narrows from the base to 

 the apex of the cochlea, divides the turns of this canal, internally, 

 into two semi-cylindrical spiral canals, named the scala?. One of 

 these, at the base of the cochlea, opens freely into the vestibule, and 

 hence is named the vestibular scala. The other, the tympanic scala, 

 ends at the fenestra rotunda of the tympanum. The two scalse, more- 

 over, communicate at the summit or cupola of the cochlea, by an open- 

 ing in the membranous part of the spiral septum, named the helico- 

 trema. 



Within the vestibule and semicircular canals are certain membran- 

 ous sacs and canals, which constitute what is named the membranous 

 labyrinth. In the vestibule are two sacs, a smaller one, globular in 

 form, the saccule, lying near the entrance into the cochlea, and a 



