ANATOMY OF THE INTERNAL EAR. 215 



completely. Yiewed externally, it appears as a single tube, 

 following the turns of the bony cochlea, beginning below, at 

 the first turn, by a blind extremity, and terminating in a 

 blind extremity at the summit of the cochlea. If we make 

 a section of the cochlea, in a direction vertical to its coils, it 

 will be seen that this canal is divided, partly by bone and 

 partly by membrane, into an inferior portion, a superior por- 

 tion, and a triangular canal, lying between the two, which is 

 external. The bony septum is in the form of a spiral plate, 

 extending from the central column, the modiolus, into the 

 cavity of the cochlea, about half-way to its external wall, 

 and terminating above in a hook-shaped extremity, called 

 the hamulus. The free edge of this bony lamina is thin 

 and dense. Near the central column, it divides into two 

 plates, with an intermediate spongy structure, in which are 

 lodged vessels and nerves. The surface of the bony lamina 

 looking toward the base of the cochlea is marked by nu- 

 merous regular, transverse ridges, or striae. 



Attached to the free margin of the bony lamina, is a 

 membrane, the membrana basilaris, which extends to the 

 outer wall of the cochlea. In this way, the canal of the 

 cochlea is divided into two portions, one above and the 

 other below the septum. The portion below begins at the 

 f enestra rotunda, and is called the scala tympani. The por- 

 tion above, exclusive of the triangular canal of the cochlea, 

 communicates with the vestibule, and is called the scala ves- 

 tibuli. 



Above the membrana basilaris, is a membrane, the limbus 

 laminae spiralis, the external continuation of which is called 

 the membrana tectoria, or the membrane of Corti. Between 

 the membrana tectoria and the membrana basilaris, is the 

 organ of Corti. The membrane of Keissner extends from 

 the inner portion of the limbus upward and outward to the 

 outer wall of the cochlea. This divides the portion of the 

 cochlea situated above the scala tympani into two portions, 

 an internal portion, the scala vestibuli, and an external, trian- 



