COCHLEA. MODIOLUS. 



515 



bone : it commences by an ampullary dilatation in the posterior ven- 

 tricle, and curves nearly perpendicularly upwards to terminate in the 

 common canal. In the ampulla of this canal are numerous minute 

 openings for nervous filaments. The inferior or horizontal canal 

 (canalis semicircularis horizontalis) is directed outwards towards the 

 base of the petrous bone, and is shorter than the two preceding. It 

 commences by an ampullary dilatation in the superior ventricle, and 

 terminates in the posterior ventricle. 



Fig. is-.* 



The COCHLEA (snail 

 shell) forms the anterior 

 portion of the labyrinth, 

 corresponding by its 

 apex with the anterior 

 wall of the petrous 

 bone, and by its base 

 with the anterior de- 

 pression at the bottom 

 of the cul de sac of the 

 meatus aiiditorius in- 

 ternus. It consists of 

 an osseous and gradual- 

 ly tapering canal, about 

 one inch and a half in 

 length, which makes 

 two turns and a half spirally around a central axis called the modiolus. 



The central axis or modiolus is large near its base where it corre- 

 sponds with the first turn of the cochlea, and diminishes in diameter 

 towards its extremity. At its base it is pierced by numerous 

 minute openings which transmit the filaments of the cochlear nerve. 

 These openings are disposed in a spiral manner: hence they have 

 received from Cotunniust the name of tractus spiralis foraminu- 



* The cochlea divided parallel with its axis, through the centre of the modi- 

 olus. After Breschet. 1. The modiolus. 2. The infundibulum in which the 

 modiolus terminates. 3, 3. The cochlear nerve, sending its filaments through 

 the centre of the modiolus. 4, 4. The scala tympani of the first turn of the 

 cochlea. 5, 5. The scala vestibuli of the first turn; the septum between 4 and 

 5 is the lamina spiralis ; a filament of the cochlear nerve is seen passing be- 

 tween the layers of the lamina to be distributed upon the membrane which in- 

 vests the lamina. 7- The membranous portion of the lamina spiralis. 8, 

 Loops formed by the filaments of the cochlear nerve on the lamina spiralis. 

 9, 9 Scala tympani of the second turn of the cochlea. 10, 10. Scala vestibuli 

 of the second turn; the septum between the two is the lamina spiralis. 11. 

 The remaining half turn of the scala vestibuli ; the dome placed over this half 

 turn is the cupola ; a line leads from the numeral to the remaining half turn of 

 the scala tympani. The lamina of bone which forms the floor of the scala 

 vestibuli curves spirally round to constitute the infundibulum (2). 14. The 

 helicotrema through which a bristle is passed ; its lower extremity issues from 

 the scala tympani of the middle turn of the cochlea. 



t Dominico Cotunnius, an Italian physician ; his dissertation " De Aquse- 

 ductibus Auris Humanae Internae" was published at Naples in 1/61. 



