HE A RING 



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divided into three well-marked parts: the cochlea, the vestibule, and 

 the semicircular canals (Fig. 436). The cochlea, the most anterior of 

 the three, consists of a convoluted tube which coils round a central 

 pillar, the columella or modiolus, like a spiral staircase. The lamina 

 spiralis projects from the modiolus and divides the tube into an upper 

 compartment, the scala vestibuli, and a lower, the scala tympani 

 (Fig. 437). The part of the lamina next the modiolus is of bone, but it 

 is completed at its outer edge by a membrane, the lamina spiralis mem- 

 branacea, or basilar membrane. The scala tympani abuts on the 

 fenestra rotunda, and its perilymph is only separated from the air of 

 the tympanic cavity by the membrane which closes that opening. 

 At the apex of the cochlea the lamina spiralis is incomplete, ending in 

 a crescentic border, so that the scala tympani and the scala vestibuli 

 here communicate by a small opening, the helicotrema. The scala 

 vestibuli communicates with the vestibule, and the vestibule with the 

 semicircular canals, so that the perilymph of the entire labyrinth forms 

 a continuous sheet separated from the cavity of the middle ear by the 



9 



Fig. 436. Diagram of Right Membranous Labyrinth (after lestut). i, utricle; 

 2, 3, 4. superior, posterior, and horizontal semicircular canals; 5, saccule; 

 6, ductus endolymphaticus arising by two branches, 7, 7'; 8, saccus endo- 

 lymphaticus; 9, canalis cochlearis (canal of the cochlea) ending at 9', and 9*; 

 10, canalis reunions. 



structures that fill up the round and oval foramina. In the mem- 

 branous labyrinth, and in it alone, are contained the end-organs of the 

 auditory nerve. The membranous portion of the cochlea is a small 

 canal of triangular section, cut off from the scala vestibuli by the mem- 

 brane of Reissner, which stretches from near the edge of the bony 

 spiral lamina to the outer wall (Fig. 438), to which it is attached by the 

 spiral ligament. The canal has received the name of the scala media, 

 or canal of the cochlea. The membrane of Reissner forms its roof. 

 Its floor is composed (i) of the projecting edge of the spiral lamina, 

 called the limbus, and (2) of tfte basilar membrane. The most con- 

 spicuous constituent of the basilar membrane is a layer of stiff, parallel, 

 transparent fibres arranged radially i.e., in the direction from limbus 

 to spiral ligament. They are embedded in a homogeneous material. 

 Below the cochlear canal ends blindly, but communicates by a side- 

 channel with the portion of the membranous vestibule called the sac- 

 cule, which in its turn communicates with the utricle by the Y-shaped 

 origin of the ductus endolymphaticus. Into the utricle open the three 



