THE INTERNAL EAR.

The bony canal of the cochlea  takes two turns and three-quarters round the modiolus. It is a little over an inch in length (about 30 mm.), and
diminishes gradually in size from the base to the summit, where it terminates in a cul-de-eac, the cupola, which forms the apex of the cochlea. The commence¬ment of this canal is about the tenth of an inch in diameter: it diverges from the modiolus toward the tympanum and vestibule, and presents three openings. One, the fenestra rotunda, communicates with the tympanum; in the recent state this aperture is closed by a membrane, the membrana tympani secundaria. Another aperture, of an elliptical form, enters the vestibule. The third is the aperture of the aqueeductus cochle~, leading to a minute funnel-shaped canal, which opens on the basilar surface of the petrous bone and transmits a small vein, and also forms a communication between the subarachnoidean space of the skull and the perilymph contained in the scala tympani.


The lamina spiralis ossea is a bony shelf or ledge which projects outward from
the modiolus into the interior of the spiral canal, and, like the canal, takes two and three-quarter turns round the modiolus. It reaches about half-way toward the outer wall of the spiral tube, and partially di "ides its cavity into two passages or scalee, of which the upper is named the scala vestibuli, while the lower is termed the scala tlj1nlJani. Near the summi t of the cochlea the lamina terminates in a hook-shaped •process, the hamulus, which assists to form the boundary of a small opening, the helicotrema, by which the two scalse communicate with each other. From the canalis spiralis modioli numerous foramina pass outward through the osseous spiral lamina as far as its outer or free edge. In the lower part of the first