568 THE AUDITORY APPARATUS 



spirally for two turns and a half around a central pillar, termed 

 the modiolus. The appearance produced is somewhat similar 

 to that of a spiral shell. The cochlea lies anterior to the 

 vestibule, with its base directed towards the bottom of the 

 internal acustic meatus ; whilst its apex is directed antero- 

 laterally, and lies in close relation with the canal for the 

 tensor tympani muscle. 



The cochlear tube rapidly diminishes in diameter as it 

 is traced towards the apex of the cochlea, and its closed 

 extremity is termed the cupola. The first turn which it 

 takes around the modiolus produces the bulging on the 

 medial wall of the tympanum, which has been described under 

 the name of the promontory. 



The modiolus is thick at the base, but rapidly tapers 

 towards the apex. Its base abuts against the bottom of 

 the internal acustic meatus. It forms the inner wall of 

 the cochlear tube, and winding spirally round it, like the 

 thread of a screw, is a thin lamina of bone, termed the lamina 

 spiralis, which partially subdivides the tube into two passages. 



Numerous minute canals traverse the modiolus, and one more con- 

 spicuous than the others, the longitudinal canal of the modiolus, extends 

 along its centre. The spiral lamina also is tunnelled by small canals 

 in communication with those in the modiolus, whilst one, the spiral 

 canal of the modiolus, winds spirally around the central pillar in the 

 attached margin of the spiral lamina. All these channels convey 

 filaments from the cochlear division of the acustic nerve to the membranous 

 cochlea, whilst the spiral canal lodges the ganglion spirale cochlea, which 

 is the peripheral ganglion of the cochlear part of the acustic nerve. 



The membranous cochlear tube or ductus cochlearis is placed 

 between the free margin of the spiral lamina and the 

 opposite side of the wall of the cochlear tube, and completes 

 the subdivision of the bony cochlea into two compartments, 

 which are termed the scala tympani and the scala vestibuli. 

 The scala tympani is the larger of the two. It begins at the 

 fenestra cochleae, where the secondary membrane of the 

 tympanum shuts it off from the tympanic cavity. At the 

 apex of the cochlea it communicates with the scala vestibuli 

 by means of an aperture, termed the helicofrema. At the 

 base of the cochlea the scala vestibuli communicates with 

 the lower and anterior part of the vestibule. The perilymph 

 therefore, in the semicircular canals and vestibule, is 

 directly continuous with that in the scala vestibuli and 

 scala tympani. 



