640 ORGANS OF THE SENSES. 



aquseductus cochlese, which leads to a minute funnel-shaped canal, which opens on 

 the basilar surface of the petrous bone, and transmits a small vein. 



The interior of the spiral canal is divided into two passages (scalse) by a thin, 

 osseous, and membranous lamina, which winds spirally round the modiolus. This 

 is the lamina spiralis, the essential part of the cochlea upon which the nerve 

 tubules are distributed. The osseous part of the spiral lamina -extends about half 

 way across the diameter of the spiral canal ; it is called the osseous zone. It 

 commences in the vestibule between the tympanic and vestibular opening of the 

 cochlea, and gradually becoming narrower in its course, terminates in a projecting 

 hook, the hamular process, just where the expansion of the infundibulum com- 

 mences. The lamina spiralis consists of two thin lamellae of bone, between which 

 are numerous canals for the passage of nervous filaments, which open chiefly 

 on the lower or tympanic surface. At the point where the osseous lamina is 

 attached to the modiolus, and following its windings, is a small canal, called by 

 Rosenthal, the canalis spiralis modioli. In the recent state, the osseous zone is 

 continued to the opposite wall of the canal by a membranous and muscular layer 

 (membranous zone), so as to form a complete partition in the tube of the cochlea. 

 Two passages or scaloe are thus formed, by a division of the canal of the cochlea 

 into two. One, the scala tympani, is closed below by the membrane of the 

 fenestra rotunda; the other, the scala vestibuli, communicates, by an oval 

 aperture, with the vestibule. Near the termination of the scala vestibuli, close 

 by the fenestra rotunda, is the orifice of the aquaeductus cochleae. The scalae 

 communicate, at the apex of the cochlea, by an opening common to both, the 

 helicotrema, which exists in consequence of the deficiency of the lamina spiralis 

 in the last half-coil of the canal. 



In structure, the membranous zone is a transparent glassy lamina, presenting 

 near its centre a number of minute transverse lines, which radiate outwards, and 

 give it a fibrous appearance ; and at its circumference, where it is connected with 

 the outer wall of the spiral canal, it is composed of a semi-transparent structure, 

 the Cochlearis muscle (Todd and Bowman), connective tissue (Kolliker). 



The vestibular surface of the osseous portion of the lamina spiralis is covered, 

 for about the outer fifth of its surface, with a thin layer, resembling cartilage in 

 texture. It is described as the denticulate lamina (Todd and Bowman), from its 

 presenting a series of wedge-shaped teeth which form its free margin, and which 

 project into the vestibular scala?. 



The inner surface of the osseous labyrinth is lined by an exceedingly thin 

 fibre-serous membrane, analogous to a periosteum, from its close adhesion to the 

 inner surface of these cavities, and performing the office of a serous membrane by 

 its free surface. It lines the vestibule, and from this cavity is continued into the 

 semicircular canals and the scala vestibuli of the cochlea, and through the heli- 

 cotrema into the scala tympani. Two delicate tubular processes are prolonged 

 along the aqueducts of the vestibule and cochlea, to the inner surface of the dura 

 mater. This membrane is continued across the fenestra ovalis and fenestra rotunda, 

 and consequently has no communication with the lining membrane of the tympa- 

 num. Its attached surface is rough and fibrous, and closely adherent to the bone ; 

 its free surface is smooth and pale, covered with a layer of epithelium, and secretes 

 a thin, limpid fluid, the aqua labyrinthi, perilymph (Blainville), or liquor Cotunnii. 

 In the vestibule and semicircular canals, it separates the osseous from the mem- 

 branous labyrinth; but in the cochlea it lines the two surfaces of the bony lamina 

 spiralis, and, being continued from its free margin across the canal to its outer 

 wall, forms the lamina spiralis membranacea, serving to complete the separation 

 between the two scalaB. 



THE MEMBRANOUS LABYRINTH. 



The Membranous Labyrinth (fig. 318) is a closed membranous sac, containing 

 fi aid ; upon the wall of the sac, the ramifications of the auditory nerve are distributed. 



