THE APPAPATL'S OF THE SEXSES. 



twisting downwards, forwards, and upwards, around a central conical axis 

 (the modiolus) ; so that its centre nearly corresponds to the inner wall of the 



tympanum. A partition the 



Fig. 392. lamina spiral is, spiral like the 



cavity divides it into two dis- 

 tinct sections or scalce : a supe- 

 rior and inferior ; this lamina is 

 attached by its inner border to 

 the central axis of the cochlea, 

 but is free at its external margin, 

 which does not quite reach the 

 periphery of the cavity. The 

 two scalse, therefore, communi- 

 cate, in the skeleton, by means 

 of an opening (the helico-trema) 

 that follows the free border of 

 the lamina spiralis throughout 



SECTION THROUGH ONE OF THE COILS OF THE 

 COCHLEA. 



ST, Scala tympani ; sv, Scala vestibuli : cc, Canalis 

 cochleae ; membrana of Reissner. Us to Isp, Lamina 

 spiralis membranacea; Us, Iambus laminae spi- 

 ralis; ss, Sulcus spiralis; gs, Ganglion spirale 

 situated on nc, the nervous cochlearis indicated 

 by the black line; /so, Lamina spiralis ossea; /, 

 Membrana tectoria; 6, Membrana basilaris; co, 

 Organ of Corti ; /sp, Ligamentum spirale ; Cc, 

 Cells of Claudius. 1, Rod of Corti of the first 

 order ; 2, Rod of Corti of the second order. 



its extent. 



The inferior scale (or scala 

 vestibuli) enters the vestibule; 

 the commencement of the supe- 

 rior scale, or scala tympani, is 

 formed by the fenestra rotunda 

 (fenestra cochlece), which brings 

 it into communication with the 

 middle ear, without the presence 

 of a membrane exactly closing 

 that aperture. 



THE MEMBRANOUS LABYRINTH. 



The membranous labyrinth comprises three parts, corresponding to the 

 three cavities of the osseous labyrinth: 1, The vestibule; 2, The semi- 

 circular canals ; 3, The cochlea. 



Fig. 393. 1. Ttie Membranous Vestibule. 



This is composed of two sacs with thin, 

 soft walls, lodged in the osseous labyrinth ; 

 the superior is the largest, is oval-shaped, 



1, Modiolus : 2, Infundibulum in which the mo- 

 diolus terminates ; 3, 3, Cochlear nerve, send- 

 ing its filaments through the centre of the 

 modiolus ; 4, 4, Scala tympani of the first 

 turn of the cochlea ; 5, 5, 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 between the 

 layers of the lamina to be distributed in the 

 membrane investing the lamina; 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 is the lamina spiralis; 11, 

 The remaining half turn of the scala vestibuli ; the dome above is the cupola, the line 

 passing through it leads to the remaining half turn of the scala tympani. The 

 osseous lamina forming the floor of the scala vestibuli curves spirally round to con- 

 stitute 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. 



SECTION OF THE COCHLEA PARALLEL TO 

 ITS AXIS, THROUGH THE CENTRE OF 

 THE MODIOLUS. 



