SENSE OF HEARING. (509 



upper is again subdivided by a membrane, the membraneofReissner, 

 into two canals : that between the inner wall of the cochlea and 

 this membrane being the scala vestibuli; and that between the 

 outer wall and the membrane of Reissner, having the membrana 

 basilaris as its base, the scala media, ductus cochlece, or canalis 

 cochlece. This latter is in reality a part of the membranous 

 labyrinth, not of the osseous, but it is somewhat more convenient 

 to describe it at this point. At the apex of the cochlea the 

 lamina spiralis ends in the hamulus, a hook-like process, and here 

 the scala vestibuli and scala tympani communicate, the opening 

 being the helicotrema. At the junction of the lamina spiralis and 

 the modiolus, and winding around the latter, is the canalis spiralis 

 modioli, which lodges the ganglion spirale, an enlargement of the 

 cochlear nerve containing ganglion-cells. From this are given 

 off the nerves to the organ of Corti. 



The scala vestibuli communicates with the vestibule at its 

 lower end. The scala tympani at its lower end terminates at the 

 fenestra rotunda, which is closed by the membrana tympani 

 secundaria. 



Aquceductus Cochlece. This is a small canal running from the 

 scala tympani to the basilar surface of the petrous bone which 

 transmits a vein from the cochlea that joins the internal jugular 

 vein. 



Lining of Osseous Labyrinth. All the cavities of the osseous 

 labyrinth are lined by a fibroserous membrane, regarded by some 

 as periosteum; this membrane also covers the fenestra ovalis 

 and fenestra rotunda. On its inner surface is a layer of epithe- 

 lium which secretes the perilymph, a watery fluid containing 

 mucin, which fills so much of the osseous labyrinth as is not 

 occupied by the membranous labyrinth. 



Membranous Labyrinth (Fig. 391). The membranous labyrinth 

 is contained within the osseous, and is, to a certain extent, a dupli- 

 cation of it. Its walls consist of three layers : 1 . An external, 

 which is made up of fibrous tissue, rather loose in structure, in 

 which are blood-vessels and pigment-cells similar to those in the 

 retinal pigmentary layer ; 2. A middle layer, thicker than the 

 external, and somewhat like the hyaloid membrane of the eye ; 

 and 3. An internal, composed of polygonal nucleated epithelium 

 which secretes the endolymph or liquor Scarpce, a fluid similar in 

 composition to the perilymph, and contained within the membra- 

 nous labyrinth, as the perilymph is within the osseous. 



The membranous labyrinth consists of the utricle and saccule, 

 which are contained in the vestibule ; the three membranous semi- 

 circular canals, and the canal of the cochlea or scala media. 



The utricle, saccule, and membranous semicircular canals are 

 attached on one side to the wall of the osseous labyrinth, and from 

 the opposite side are given off bands of fibrous tissue which hold 



39 



