the 



DUCTUS COCHLEAEIS. 



849 



Outer attachment of 

 the membrana 

 vestibularis 



Stria vascularis 



apex of the latter its upper extremity, the lagena, or caecum capulare, is 

 fixed to the cupula and partly bounds the helicotrema. As already stated, the 

 membrana basilaris extends from the free edge of the lamina spiralis ossea to the 

 outer wall of the cochlea. A second, more delicate membrane, the membrana 

 vestibularis (O.T. membrane of Reissner), stretches from the thickened periosteum 

 covering the upper surface of the lamina spiralis ossea to the outer cochlear wall, 

 some little distance above the external attachment of the membrana basilaris. A 

 canal is thus enclosed between the underlying scala tympani and the overlying 

 scala vestibuli, and constitutes the ductus cochlearis. Triangular on transverse 

 section, the duct possesses a roof, an outer wall, and a floor, and is lined throughout 

 with epithelium and filled with endolymph. On its floor the epithelium is greatly 

 modified, and there the endings of the cochlear nerve are found. 



The roof or vestibular wall of the ductus cochlearis is formed by the mem- 

 brana vestibularis, a delicate, nearly homogeneous membrane, covered on each 

 surface by a layer of epithelium. Its entire thickness is about 3 p. 



The outer wall of the ductus cochlearis (Fig. 724) consists of the periosteal 

 lining of the bony cochlea, which, however, is thickened and greatly modified to 

 form the ligamentum spirale cochleae. Occupying the whole outer wall, this liga- 

 ment projects inwards inferiorly as a 

 triangular prominence, the crista basilaris, 

 to which the outer edge of the membrana 

 basilaris is attached. In the upper part 

 of the ligamentum spirale the periosteum 

 is of a reddish-yellow colour, and con tains, 

 immediately under its epithelial lining, 

 numerous small blood-vessels and capil- 

 lary loops, forming the stria vascularis. 

 The lower limit of this stria is bounded 

 by a prominence, the prominentia spiralis, 

 in which is seen a vessel, the vas pro- 

 minens, and between this prominence 

 and the crista basilaris is a concavity, 

 the sulcus spiralis externus. The height 

 of the outer wall diminishes towards the 

 apex of the cochlea. 



The floor or tympanal wall of the 

 ductus cochlearis is formed by the peri- 

 osteum covering that portion of the 

 lamina spiralis ossea which is situated 

 to the outer side of the membrana ves- 

 tibularis, and by the membrana basilaris, 

 which stretches from the free edge of 

 the lamina spiralis ossea to the crista 

 basilaris. On the inner part of the 

 membrana basilaris the complicated 

 structure termed the organon spirale 

 (O.T. organ of Corti) is situated. The 

 lamina spiralis ossea consists of two 

 plates of bone, between which are placed 

 the canals for the branches of the cochlear 

 nerve. On the upper plate the perios- 

 teum is thickened and modified to form 

 the limbus laminae spiralis, the outer ex- 

 tremity of which forms a C-shaped con- 

 cavity, the sulcus spiralis interims. The 

 portions of the limbus which project 

 above and below this concavity are 

 ermed respectively the labium vestibulare and labium tympanicum. The latter is 

 perforated by about 4000 small apertures, the foramina nervosa, for the transmission 



55 



FIG. 724. TRANSVERSE SECTION THROUGH OUTER 

 WALL OF DUCTUS COCHLEARIS (Schwalbe). 



