THE ORGAN OF HEARING. 389 



merit the perilymphatic space, instead of constituting a single 

 cavity in which the epithelial tube is suspended, is divided into the 

 scala vestibuli above and the scala tympani below, which com- 

 municate respectively with the vestibule and the tympanum. 



The ductus cochlearis, or scala media, consists, like other por- 

 tions of the membranous labyrinth, of the epithelial tube, the 

 oldest part of the cochlea representing the specialized outgrowth 

 from the primary ectodermic otic vesicle, and the supporting fibrous 

 tunic derived from the differentiated surrounding mesoderm. 



The ductus cochlearis is triangular in section ; its base or ex- 

 ternal wall is attached to the outer wall of the bony capsule, its 

 apical border is joined to the end of the osseous spiral lamina, 

 and the converging sides are formed by the delicate membrane of 

 Reissner above and the basilar membrane below, which separate 

 respectively the scala vestibuli and the scala tympani from the scala 

 media. 



The vestibular wall of the cochlear duct is formed by Reiss- 

 ner 's membrane, an extremely fragile partition dividing the duct 

 from the scala vestibuli ; the membrane begins on the vestibular 

 border of the lamina spiralis, about .2 mm. inside the free edge of 

 the crista, and extends at an angle of about forty-five degrees until 

 it meets the outer bony wall. Reissner's membrane consists of 

 three layers : an extremely thin central, almost homogeneous 

 connective-tissue stratum, one side of which is covered by the 

 endothelium of the vestibular surface and the other by the epi- 

 thelium of the cochlear duct. Notwithstanding the extreme thin- 

 ness of this layer, the presence within it of sparingly distributed 

 capillary blood-vessels has been demonstrated. The vestibular 

 endothelium consists of a single layer of delicate plates, which here 

 and there enclose pigment. The surface towards the duct is cov- 

 ered by the general ectodermic lining of the canal, represented by 

 a single layer of flat polyhedral epithelial cells. The three 

 layers contribute equally to the 3 M representing the entire thickness 

 of the membrane. 



The outer wall of the cochlear duct rests against a greatly thick- 

 ened crescentic cushion of connective tissue, whose convex 

 border is intimately united with the bony wall, and whose generally 

 concave margin looks towards the cochlear duct. This area of con- 

 nective tissue, the ligamentum spirale, extends both above and 

 below the boundary of the cochlear duct, its two horns forming part 

 of the outer walls of the adjacent vestibular and tympanic canals. 



The concave surface of the ligamentum spirale is interrupted 

 opposite the level of the tympanic wall of the cochlear duct by a pro- 

 jecting ridge, the crista basilaris (Schwalbe), to which the basilar 



