THE INTERNAL EAR. 



375 



spiral is ossea to the outer wall of the cochlea, where it is attached to the periosteum. 

 Notwithstanding its excessive thinness (.3 /"), it consists of three layers : (a) a very 

 delicate middle stratum of connective tissue, (b} the endothelium covering the vestibu- 

 lar side, and (c) the epithelium derived from the ectodermic cochlear duct. It also 

 contains sparingly distributed capillary blood-vessels. 



The outer wall of the cochlear duct (Fig. 423) is bounded by a part of a thick- 

 ened crescentic cushion of connective tissue, whose convex surface is closely united 

 with the bony wall and whose generally concave surface looks towards the cochlear 

 duct. This structure, the ligamentum spirale, extends slightly above the attachment 

 of Reissner's membrane and to a greater distance below the attachment of the basilar 

 membrane, thus forming part of the outer walls of the scalae vestibuli and tympani. 

 At its junction with the basilar membrane it presents a marked projection, the crista 

 basilaris, while a very slight elevation marks the point of attachment of the membrane 



Bony capsule 

 of cochlea 



tria vascularis 



Reissner's membrane 



Prominentia spiralis 

 Membrana tectoria 



Spiral ligament 



Crista basilaris 



Nerve-fibres 



Vas spiralis 



FIG. 423. Part of section ot human cochlea, showing cochlear duct with Corti's organ. 

 (Preparation by Dr. Ralph Butler.) 



X9- 



of Reissner. The part of this ligament lying between these projections corresponds 

 to the outer wall of the cochlear duct. Its concave free inner surface is broken by a 

 third elevation, the prominenlia spiralis, or accessory spiral ligament, distinguished 

 usually by the presence of one large (vas prominens) or several small blood-vessels. 

 The lower and smaller of these two divisions of the outer wall is called the sulcus 

 spiralis externus and is lined by cuboidal epithelium, while the larger upper division 

 is occupied by a peculiar vascular structure, the stria vascularis, which contains capil- 

 lary blood-vessels within an epithelial structure. Its surface is covered with pigmented 

 irregular polygonal epithelial cells, and its deeper strata consist of cells which, espe- 

 cially in the superficial layers, resemble the surface epithelium, but in the deeper layers 

 assume more and more the character of connective tissue. Over the prominentia 

 spiralis the cells become flat and polyhedral. 



The ligamentum spirale is composed of a peculiar connective tissue, rich in cells 

 and blood-vessels. Its thin outer layer forms the periosteum and is denser than the 

 adjacent loose connective tissue. The latter is broadest opposite the scala tympani, 

 where its fibres converge towards the crista basilaris. Opposite the outer wall of the 

 cochlear duct it again becomes more compact and is rich in cells and blood-vessels. 

 An internal layer extending from near the prominentia spiralis to the basilar membrane 

 consists of a hyaline noncellular tissue. Some authors claim to have found smooth 

 muscle-fibres in the ligamentum spirale. 



