THE EAR 



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cupola it ends in a blind- extremity. It gradually diminishes in 

 size from base to cupola; its length is about 32 millimetres; and 

 its diameter is about 2 millimetres at the base, where it is greatest. 

 The modiolus is the central pillar round which the spiral canal 

 of the cochlea turns, and it forms the inner wall of that canal. 

 It commences at the cochlear area of the lamina cribrosa at the deep 

 end of the internal auditory meatus, and extends almost to the 

 cupola, gradually tapering. It is traversed by minute canals for 

 branches of the cochlear division of the auditory nerve. One of 

 these canals occupies the centre of the modiolus, and is called the 

 central canal of the modiolus. This canal commences at the 

 foramen centrale of the cochlear area of the lamina cribrosa, and it 

 transmits the nerve-filaments for the apical coil. The other canals, 

 which have no special name, commence at the tr actus spiralis 



Fig. 650. — Median Section of the Left Osseous Cochlea of Man 

 FROM Apex to Base (Arnold). 



S.V. Scala Vestibuli L.S.O. Lamina Spiralis Ossea 



S.T. Scala Tympani C.C. Central Canal of Modiolus 



M.A.I. Meatus Auditorius Internus 



foraminosus of the cochlear area of the lamina cribrosi, and they 

 transmit the nerve-filaments for the other coils — middle and 

 basal. At successive levels these canals change their direction, 

 and pass outwards to the attached margin of the lamina spiralis, 

 to be presently described. Here they coalesce and form a winding 

 canal, called the spiral canal of the madiolus, which lodges the 

 spiral ganglion or ganghon of Corti. From this canal secondary 

 canals for nerve-filaments pass into the lamina spiralis. 



The lamina spiralis ossea is a thin plate of bone, which winds 

 spirally round the modiolus, to which it is attached. It projects 

 from the modiolus into the spiral canal of the cochlea throughout 

 the windings of the latter, and it extends for about half-way towards 

 the outer wall of the cochlear canal. It divides that canal incom- 

 pletely into two passages or scal-cB — ^aa upper, or scala vestibuli. 



