I5i8 HUMAN ANATOMY. 



scala vestibuli and communicates with the perilymphatic space of the vestibule. The 

 scalae tympani and vestibuli communicate only at the apex of the cochlea through 

 the helicotrema. They contain perilymph and are brought into relation with the 

 subarachnoid space through the aquaeductus cochleae. They are lined by a delicate 

 fibrous periosteum, usually covered on the surface which is in contact with 

 the enclosed perilymph, by a single layer of endothelial plates. In some localities, 

 however, as on the tympanic surface of the basilar membrane, the lining cells 

 retain their primitive mesoblastic 'character and never become fully differentiated into 

 endothelium. 



The third compartment, the ductus cochlearis, is triangular on cross-section 

 (Fig. 1 271), except at its ends, and bounded by Reissner's membrane above, by 

 the basilar membrane and a part of the osseous spiral lamina below, and by the 

 outer wall of the bony cochlea externally. Save for the narrow channel, the 

 canalis reuniens, by which it communicates with the saccule, the cochlear duct is 

 a closed tube and contains endolymph. It begins below as a blind extremity, the 

 caecum vestibulare, lodged within the recessus cochlearis of the vestibule and, 

 after making two and three-quarter turns through the cochlea, ends above at the 

 cupola of the cochlea in a second blind extremity, the caecum cupulare, or 

 lagena, which is attached to the cupola and forms a part of the boundary of the 

 helicotrema. 



Architecture and Structure of the Cochlear Duct. — ^Reissner's membrane (membrana vestib- 

 ularis), the delicate partition separating the cochlear duct from the scala vestibuli, begins on 

 the upper surface of the lamina spiralis, about .2 mm. medial to the free edge of the bony 

 shelf, and extends at an angle of from 40-45° with the lamina spiralis ossea to the outer wall of 

 the cochlea, where it is attached to the periosteum. Notwithstanding its excessive thinness 

 (.003 mm.), it consists of three layers : (a) a very delicate middle stratum of connective tissue, 

 [b) the endothelium covering the vestibular side, and (r) the epithelium derived from the coch- 

 lear duct, and contains sparingly distributed capillary blood-vessels. 



The outer wall of the cochlear duct (Fig. 1272) is bounded by a part of a thickened cres- 

 centic cushion of connective tissue, whose convex surface is closely united with the bony wall 

 and whose generally concave surface looks toward the cochlear duct. This structure, the liga- 

 mentum 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, whilst a very slight elevation marks the point of attach- 

 ment of the membrane 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 prominentia 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 Guboidal epithelium, whilst the larger upper division is occupied by a peculiar vascular 

 structure, the stria vascularis, which contains capillary blood-vessels w-ithin an epithelial struc- 

 ture. Its surface is covered with pigmented irregular polygonal epithelial cells, and its deeper 

 strata consist of cells which, especially 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 con- 

 nective 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. 



The tympanic wall or floor of the cochlear duct (Fig. 1272) comprises the basilar mem- 

 brane, extending from the basilar crest to the outer end of the bony spiral lamina, and the limbus 

 lamincc spiralis, which includes this wall from the attachment of Reissner's membrane to the end 

 of the bony lamina. The limbus (crista spiralis) is a thick mass of connective tissue upon the 

 upper surface of the outer end of the osseous lamina spiralis. Its outer extremity is deeply 

 grooved to form a gutter, the sulcus spiralis internus, the projections of the limbus above and 

 below the sulcus forming respectively its superior (vestibular) and inferior (tympanic) labia. 

 The upper surface of the limbus is marked by clefts and furrows which are most conspicuous 

 near the outer margin of the upper lip (labium vestibulare), where the irregular projections between 



