INTERNAL EAR. 705 



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 corre- 

 sponds with the anterior depression at the bottom of the internal auditory 

 meat us ; 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 spirallv 

 round the axis for two turns and a half, from the base to the apex ; and of a 

 delicate lamina (the 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 

 the cochlea, and is perforated with 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 wall of the spiral canal, and is dense in structure; but its 

 centre is channelled, as far as the last half-coil, by numerous branching canals, 

 which transmit nervous filaments in regular succession into the canal of the 

 cochlea, or on to the surface of the lamina spiralis. One of these, larger than 

 the rest, occupies the centre of the modiolus, and is named the tubnlus centralis 

 modioli ; it extends from the base to the extremity of the modiolus, and trans- 

 mits a small nerve and artery (arteria centralis modioli). 



The spiral canal (Fig. 382) 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 



Fig. 382. The Cochlea laid open. (Enlarged.) 



diminishes 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 com- 

 mencement 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 aquaeductus cochleae leading to a minute funnel-shaped 

 canal, which opens on the basilar surface of the petrous bone, and transmits a 

 small vein. 



The interior of the spiral canal is divided into two passages (scalae) by a thin, 

 osseous, and membranous lamina, which winds spirally round the modiolus. 

 This is the lamina spiralis, the essential part of the cochlea upon which the 

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