THE AUDITORY APrARATUS. 



949 



SO that its centre nearly corresponds to the inner wall of the tympanum. A 

 partition — the litnuna spiralis, spiral like the cavity — divides it into two distinct 

 sections, or scala — a superior and inferior. This partition 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 scalae, therefore, 



Fig. 513. 



SECTION THROUGH ONE OK THE COILS OF THE 

 (i'OCHLEA. 



iST, Scala tympani ; .9F. srala vestibuli ; CC, canalis 

 cochleae; inenibraDa of Reissner ; Us to Isp, lamina 

 spiralis membranacea; Us, limbus lamina? spiralis; 

 ss, sulcus spiralis ; gs, ganglion spirale situated 

 on nc, the nervous cochlearis indicated by the 

 black line ; tso, lamina spiralis ossea ; t, mem- 

 brana tectoria; 6, membrana basilaris ; Co, organ 

 of Corti; Isp, ligamentiim spirale; Cc, cells of 

 Claudius. 1, Rod of Corti of the first order ; 2, 

 rod of Corti of the second order. 



communicate, in the skeleton, by means of an 

 opening (the helico-trema) that follows the free 

 border of the lamina spiralis throughout its 

 extent. 



The inferior scala (or scala vestilndi) enters 

 the vestibule ; the commencement of the 

 superior 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 ex- 

 actly closing that aperture. 



Si.Cllii.N <)l THK (JUCIh.l.A I'ARALLKL TO 

 ITS AXIS, THROUGH THE CENTRE OF 

 THE MODIOLUS. 



1», Modiolus; 2, infundibulum in which 

 the modiolus terminates ; 3, 3, coch- 

 lear nerve, sending its filaments 

 through the centre of the modiolus; 

 4, 4, scala tympani of the first turn 

 of the cochlea ; 5, 5, scahi vestibuli 

 of the first turn ; the se]itum 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 

 cochleiir nerve on the lamina spiralis; 

 9, 9, scala tyni|iani 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 ; tiie 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 spirnlly 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. 



The Membranous Labyrinth. 



The membranous labyrinth comprises three parts, corresponding to the three 

 cavities of the osseous labyrinth. 1. The vestibule. 2. The semicircular canals. 

 3. The cochlea. 



1. The Membranous Vestibule (Fig. 513). 



This is composed of two sacs with thin, soft walk, lodged in the osseous 

 labyrinth. The superior is the largest, is oval-shaped, and is named the utriculus ; 

 it communicates with the semicircular canals, of which it is a confluent. The 



