COMPOSITION OF SEPTUM COCHLEAE. 



683 



the axis. The surface of the lamina which is turned to the lower of the 

 two cochlear passages (ST) is most pierced by apertures. The opposite 

 surface is covered in the outer fifth of its extent by a border or limbus of 

 fibrous structure (a), which ends in wedge-shaped teeth near the margin 

 of the bony plate (denticulate lamina, Bowman). 



Fig. 247. 



A DIAGRAM OP A SECTION OF THE TUBE OF THE COCHLEA ENLARGED (modified from Henle). 



SV. Scala vestibuli. 

 ST. Scala tympani. 

 CC. Canal of the cochlea. 



1. Membrane of Keissner. 



2. Cochlear branch of the auditory nerve. 



3. Lamina spiralis ossea. 



4. Ligamentum spirale. 

 a. Lamina denticulata. 



&. Sulcus spiralis. 



c. Tympanic lip of the sulcus spiralis. 



d. Inner rods of Corti. 



e. Outer rods of Corti. 

 /. Lamina reticularis. 

 i. Inner hair cells. 



m b. Membranabasilai'is. 

 m c. Membrane of Corti. 



p. Outer hair cells. 

 8 m. Central space between the rods. 



Between the teeth and the underlying bone is a channel (b), lined by 

 flat epithelium, which is called sulcus spiralis : its edges are named ves- 

 tibular () and tympanic (c). 



The membranous part of the septum (membrana basilaris, fig. 247, m b) 

 reaches from the lower edge (c) of the lamin* spiralis to the groove in the 

 outer wall of the cochlear tube. Its width varies : near the base of the 

 cochlea it forms half of the partition across the tube, but at the apex, 

 where the bony part is wanting, it constructs the septum altogether. 



This membrane is firm and stiff, and possesses fine radiating lines which 

 cause it to split in shreds. At its outer attachment there is a fibrous band 

 ( 4 ) (ligamentum spirale, Kolliker) fixing it to the bone. 



Scala of the cochlea (fig. 246). The tube of the cochlea is divided by 

 the septum into two primary passages, of which one is the scala tympani 

 (ST) and the other scala vestibuli (sv); but the latter is rendered smaller 

 by two canals being cut off from it by membranes. 



These passages are placed one above another, the scala vestibuli (sv) 

 being the nearest the apex of the cochlea. Above they communicate 

 through the aperture named helico-trema (/). Below they end differently, 



