AUDITORY ORGAN. 3ti7 



(2) The Cochlea. 



The membranous cochlea, or ductus cochlearis, is a long 

 sac which fills up only a small part of the bony cochlea and 

 follows the spiral turns two and three-quarters times. The 

 ductus cochlearis (Fig. 269) lies between the perilymphatic 

 sacs, the scala vestibuli and scala tympani. It touches on the 

 former with its upper wall the membrana vestibuli (Reissneri] ; 

 and on the latter with its lower wall the lamina spir<t/i.< 

 membranacea. 



For convenience in description, we shall consider the cochlea 

 as not lying horizontally, but with its axis vertical, so that its 

 base is down and its apex up. If then an axial section be 

 taken, the ductus cochlearis has in cross-section a triangular 

 outline. Two of these sides form the upper and lower walls, 

 while the outer wall lies against the periosteum of the outer 

 bony cochlea. The periosteum is here distinctly thickened, 

 so that it forms on cross-section a semilunar mass of connective 

 tissue (ligainentum spirale). The angle where the upper 

 (vestibular) and lower (tympana!) walls come together lies at 

 the apex of the triangle opposite the external wall, in the 

 region of the outer free border of the lamina spiralis ossea. 

 At this point the connective tissue forms a projection on the 

 lamina spiralis ossea, the limbus spiralis. This begins at the 

 attachment of Reissner's membrane, and forms a ridge pro- 

 truding into the lumen of the ductus cochlearis. This is called 

 the labium veslibulare. Farther outward there is a process 

 which overhangs the free border of the lamina spiralis ossea 

 and lies on the wall of the scala tympani. This is the 

 tympanicum. Between these two labia there is the 

 spiralis inter nm. 



The walls of the membranous cochlea consist of a very 

 fine connective-tissue sheath and an epithelial layer. The latter 

 lines the inner surface of the ductus cochlearis and shows some 

 peculiarities in structure in certain places. The outer and upper 

 walls are formed quite simply. The membrana vesti&ufariS) 

 which forms the upper wall of the ductus cochlearis, is a very 



