SCALA TYMPANI ET VESTIBULI. 459 



lentus. The modiolus is every where traversed in the direction 

 of its length by minute canals, which proceed from the tractus 

 spiralis foraminulentus, and terminate upon the sides of the modiolus, 

 by opening into the canal of the cochlea or upon the surface of its 

 lamina spiralis. The central canal of the tractus spiralis foraminu- 

 lentus is larger than the rest, and is named the tubulus centralis 

 modioli ; it is continued onwards to the extremity of the modiolus, 

 and transmits a nerve and a small artery, the arteria centralis 

 modioli.- 



The interior of the canal of the cochlea is partially divided into 

 two passages (scalse) by means of a thin and porous lamina of bone 

 (zonula ossea laminae spiralis), which is wound spirally around the 

 modiolus in the direction of the canal. This bony septum extends 

 for about two-thirds across the diameter of the canal, and in the 

 fresh subject is prolonged to the opposite wall by means of a mem- 

 branous layer, so as to constitute a complete partition the lamina 

 spiralis. The osseous lamina spiralis consists of two thin lamellae 

 of bone, between which, and through the perforations on their sur- 

 faces, the filaments of the cochlear nerve reach the membrane of 

 the cochlea. At the apex of the cochlea the lamina spiralis termi- 

 nates in a pointed, hook-shaped process, the hamulus laminae spiralis. 

 The two scalae of the cochlea which are completely separated 

 throughout their length in the living ear, communicate superiorly 

 over the hamulus laminae spiralis by means of an opening common 

 to both, which has been termed by Breschet helico-trema (s'Xif, JXfow 

 volvere r^a). Inferiorly, one of the two scalae, the scala vestibuli, 

 terminates by means of an oval aperture in the anterior ventricle 

 of the vestibule ; while the other, the scala tympani, becomes some- 

 what expanded, and opens into the tympanum through the fenestra 

 rotunda (fenestra cochleae). Near to the termination of the scala 

 tympani is the small opening of the aquasductus cochleae. 



The internal surface of the osseous labyrinth is lined by a fibro- 

 serous membrane, which is analogous to the dura mater in perform- 

 ing the office of a periosteum by its exterior, whilst 'it fulfils the 

 purpose of a serous membrane by its internal layer, secreting a 

 limpid fluid, the aqua labyrinthi (liquor Cotunnii), and sending a 

 reflection inwards upon the nerves distributed to the membranous 

 labyrinth. In the cochlea the membrane of the labyrinth invests 

 the two surfaces of the bony lamina spiralis, and being continued 

 from its border across the diameter of the canal to its outer wall, 

 forms the membranous lamina spiralis and completes the separation 

 between the scala tympani and scala vestibuli. The fenestra ovalis 

 and fenestra rotunda are closed by an extension of this membrane 

 across them, assisted by the membrane of the tympanum and a 

 proper intermediate layer. Besides lining the interior of the osseous, 

 cavity the membrane of the labyrinth sends two delicate processes 

 along the aqueducts of the vestibule and cochlea to the internal sur- 

 face of the dura mater of the petrous portion of the temporal bone, 

 with which they are continuous. These processes are the remains 



