516 SCALA TYMPANI ET VESTIBULI. 



lentus. The modiolus is everywhere 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 foraminulentus 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 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 membranous 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 surfaces, the filaments of the cochlear 

 nerve reach the membrane of the cochlea. At the apex of the cochlea 

 the lamina spiralis terminates by a pointed, hook-shaped process, the 

 hamulus laminae spiralis. The two scalae of the cochlea, which are com- 

 pletely 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 lielico-tremu 

 (?^|, Ix'tffffu volvere rgvpa). 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 somewhat expanded, and opens into the tympanum 

 through the fenestra rotxinda (fenestra cochleae). Near the termi- 

 nation of the scala tympani is the small opening of the aquaeductus 

 cochleae. 



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

 scrous membrane^ which is analogous to the dura mater in performing 

 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 mem- 

 brane of the tympanum and a proper intennediate layer. Besides 

 lining the interior of the osseous cavity, the membrane of the laby- 

 rinth sends two delicate processes along the aqueducts of the vestibule 

 and cochlea to the internal surface of the dura mater, with which they 

 are continuous. These processes are the remains of a communication 



