720 | THE ORGANS OF SENSE. 
above the external attachment of the membrana basilaris. A canal is thus enclosed 
between the underlying scala tympani and the overlying scala vestibuli, and con- 
stitutes the membranous cochlea or ductus cochlearis. Triangular on transverse 
SCALA VESTIBULI 
Reissner’s membrane 
Stria 
vascularis 
Membrana tectoria DUCTUS COCHLEARIS ay 
Sulcus spiralis : Wg NM Al NaH 
internus Ae A 
Limbus laminze Go a 9/  Ligamentum 
spiralis fo f ANA) spirale 
/ Hy, 
ff LE i; (iy, 
oP Pian, Raa Arata Mel Sulcus spiralis 
at he st Se a 7 SIs 7 nd 
Cochlear 
nerve fibres 
Inner hair cell 
Outer hair cells 
\ rana basilaris 
SCALA TYMPANI Tembrana basilaris 
Fic. 526.—SgcTION ACROSS THE DucTuUS CocHLEARIS (Retzius). 
section, it possesses a roof, outer wall, and floor, and is lined throughout with 
epithelium and filled with endolymph. On its floor the epithelium is ereatly modified, 
and here are found the endings of the cochlear division of the auditory nerve. 
The roof or vestibular wall of the ductus cochlearis is formed by the mem- 
brane of Reissner, which consists of a delicate, nearly homogeneous membrane, 
covered on its two surfaces by a layer of epithelium. Its entire thickness is 
about 3 p. 
The outer wall of the ductus cochlearis (Fig. 527) consists of the periosteal 
lining of the bony cochlea, which, however, is much thickened and greatly modified 
to form what is termed the ligamentum spirale cochlee. Occupying the whole outer 
wall, this hgament projects inwards inferiorly as a triangular prominence, the crista 
basilaris, to which the outer edge of the membrana basilaris is attached. ‘The fibres 
of the membrane radiate into the ligament in the form of a series of bundles 
analogous to the ligamentum pectinatum iridis. In the upper part of the lga- 
mentum spirale the periosteum is of a reddish-yellow colour and contains, immedi- 
ately under its epithelial lining, numerous small blood-vessels and capillary 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 prominens, 
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 periosteum 
covering that portion of the lamina spiralis ossea which is situated to the outer side 
of Reissner’s membrane, 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 organ of Corti is placed. 
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 periosteum 
is thickened and modified to form the limbus lamine spiralis, the outer extremity 
of which forms a C-shaped concavity, the sulcus spiralis internus. The portions of 
the limbus which project outwards, above and below this concavity, are termed 
respectively the labium vestibulare and labium tympanicum. The latter is perforated 
by some 4000 small apertures, the foramina nervosa or habenula perforata, for the 
transmission of the cochlear nerves. Externally it becomes continuous with the 
