THE ORGANS OF SPECIAL SENSE 585 



to its blind ending in the cupola. It is triangular in shape on trans- 

 verse section, thus allowing a division of its walls into upper, outer, 

 and lower (Fig. 389, Dc). 



The upper or vestibular wall is formed by the thin membrane of 

 Reissner (Fig. 389, h) which separates the cochlear duct from the 

 scala vestibuli. The membrane consists of a thin central lamina of 

 connective tissue covered on its vestibular side by the vestibular 

 endothelium, on its cochlear side by the epithelium of the cochlea. 



The outer wall of the cochlear duct is formed by the spiral liga- 

 ment, which is a thickening of the periosteum. The outer part of the 

 spiral ligament consists of dense fibrous tissue, its projecting part of 

 more loosely arranged tissue. From it, two folds project slightly 

 into the duct. One, the crista basilar is (Fig. 389, r), serves for the 

 attachment of the membranous spiral lamina; the other, the spiral 

 ■prominence (Fig. 389, /"), contains several small veins. Between the 

 two projections is a depression, the external spiral sulcus (Fig. 389, v). 

 That part of the spiral ligament between the spiral prominence and 

 the attachment of Reissner's membrane is known as the stria vascu- 

 laris (Fig. 389, z). It is lined with granular cuboidal epithelial cells, 

 which, owing to the absence of a basement membrane, are not sharply 

 separated from the underlying connective tissue. For this reason the 

 capillaries extend somewhat between the epithelial cells, giving the 

 unusual appearance of a vascular epithehum. 



The lower or tympanic wall of the cochlear duct has an extremely 

 complex structure. Its base is formed by the already mentioned 

 bony and membranous division-wall between the scala media and the 

 scala tympani (bony spiral lamina and membranous spiral lamina). 



The bony spiral lamina has been described (page 583). 



The membranous spiral lamina consists of a substantia propria or 

 basilar membrane, its tympanic covering, and its cochlear covering. 



The basilar membrane (Fig. 389) is a connective-tissue membrane 

 composed of fine straight fibres which extend from the bony spiral 

 lamina to the spiral ligament. Among the fibres are a few connective- 

 tissue cells. On either side of the fibre layer is a thin, apparently 

 structureless membrane. 



The tympanic covering of the basilar membrane consists of a thin 

 layer of connective tissue — an extension of the periosteum of the spiral 

 lamina— covered over by a single layer of flat endothelial cells. 



The cochlear covering of the basilar membrane is epithelial. Owing 

 to the marked difference in the character of the epithelium, the basilar 



