THE SENSES. 279 



terior crus opens between the common orifice of the two vertical canals and 

 the inferior (ampulla) of the inferior semicircular canal. 



514. 3. The Cochlea, a twisted canal, like a snail-shell, of 

 four lines axial diameter, occupies the most anterior space of the 

 internal ear, is situated internally and before the tympanum with 

 its basis turned to the base ofmeatus auditorius internus, the apex 

 towards tuba Eustachii. The canal, canalis spiralis, consists of 

 two and a half spiral turns, the length of which together measures 

 one inch, commences with its wide basis behind the promontorium, 

 and contracts inwards towards the apex (cupola), under which it 

 terminates in a funnel-shaped cavity (scyphus). The spirals pass 

 in the right ear to the right, in the left towards the left. The ca- 

 vity of the canal is divided by a thin septum, spiral plate, from the 

 basis to the apex into a superior ladder (scala vestibuli), and into 

 an inferior (scala tympani). 



The spiral plate (lamina spiralis) follows the windings about 

 the axis of the Cochlea, to which it is attached by its internal bor- 

 der, whilst the external border is attached to the circumference of 

 the Cochlea. It consists of an osseous portion zonula ossea, which 

 occupies the internal half attached to the axis and predominates in 

 the first turn, and of a membranous, external half, zona Valsalvce. 

 The osseous lamina terminates in the third turn with a kind of 

 hook, hamulus s. rostrum [where the scales, communicate, canalis 

 scalaris communis, Cassebohm ; Helicotrema, Breschet], and it- 

 self consists of two plates, between which many canal iculi for the 

 nerves of the Cochlea are found. 



The axis, spindle, modiolus s. columella, is the porous mass of 

 bone which fills up the space in the axis of the Cochlea, bounded 

 by the concavity of the spirals. Its basis, perforated by many spi- 

 rally arranged foramina (tractus foraminulentus, for the nerv. 

 Cochlea], rests upon the base of the meatus auditor, internus ; 

 above the first turn the mass is rather thickened, and is then called 

 columella, and terminates as lamina terminalis, of a funnel shape 

 in the half turn. The external surface of the modiolus is provided 

 with two screw-like grooves which communicate through their nu- 

 merous foramina with the same number of canaliculi in the inte- 

 rior of the modiolus, the largest of which in the centre, canalis 

 centralis modioli, passes from the basis to the lamina terminalis. 



The Scala? : 



a. Scala vestibuli, is wider than b, lies more anteriorly and 

 above and opens into the vestibule (aditus ad cochleam.) 



b. Scala tympani, lies more towards the basis and opens through 

 the fenestra rotunda (clausa) into the tympanum. Both scalse 



