COCHLEA. G39 



directed outwards and backwards. Its ampullated end corresponds to the upper 

 and outer angle of the vestibule, just above the fenestra ovalis; its opposite end 

 opens by a distinct orifice at the upper and back part of the vestibule. 



The Cochlea bears some resemblance to a common snail-shell ; it forms the 

 anterior part of the labyrinth, is conical in form, and placed almost horizontally 

 in front of the vestibule ; its apex is directed forwards and outwards towards the 

 upper and front part of the inner wall of the tympanum ; its base corresponds 

 with the anterior depression at the bottom of the internal auditory meatus ; and 

 is perforated by numerous apertures, for the passage of the cochlear branch of the 

 auditory nerve. It measures about a quarter of an inch in length, and its breadth 

 towards the base is about the same. It consists of a conical-shaped central axis, 

 the modiolus or columella; of a canal wound spirally round the axis for two 

 turns and a half, from the base to the apex ; and of a delicate lamina (lamina- 

 spiralis) contained within the canal, which follows its windings, and subdivides it 

 into two. 



The central axis or modiolus is conical in form, and extends from the base to 

 the apex of the cochlea. Its base is broad, corresponds with the first turn of tho 

 cochlea, and is perforated by numerous orifices, which transmit filaments of 

 the cochlear branch of the auditory nerve ; the axis diminishes rapidly in size in 

 the second coil, and terminates within the last half-coil or cupola, in an expanded, 

 delicate, bony lamella, which resembles the half of a funnel, divided longitudinally, 

 and called the infundibulum ; the broad part of this funnel is directed towards 

 the summit of the cochlea, and blends with the last half-turn of the spiral canal 

 of the cochlea, the cupola. The outer surface of the modiolus is formed of the 

 w r all of the spiral canal, and is dense in structure ; but its centre is channelled, aa 

 far as the last half-coil, by numerous branching canals, which transmit nervous 

 filaments in regular succession into the canal of the cochlea, or upon the surface 

 of the lamina spiralis. One of these, larger than the rest, occupies the centre of 

 the modiolus, and is named the tubulus centralis modioli; it extends from the base 

 to the extremity of the modiolus, and transmits a small nerve and artery (arteria 

 centralis modioli). 



The spiral canal (fig. 317) takes two turns and a half round the modiolus. It 

 is about an inch and a half in length, measured along its outer wall ; and diminishes 



Fig. 317. The Cochlea laid open. (Enlarged.) 



gradually in size from the base to the summit, where it terminates in a cul-de- 

 sac, the cupola, which forms the apex of the cochlea. The commencement of 

 this canal is about the tenth of an inch in diameter ; it diverges from the modiolus 

 towards 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 oval form, enters the vestibule. The third is the aperture of the 



