530 THE AUDITORY APPARATUS 



the medial wall of the tympanum which has been described 

 under the name of the promontory (Figs. 223, 209). 



The modiolus is thick at the base, but rapidly tapers 

 towards the apex. Its base abuts against the bottom of the 

 internal acoustic meatus. It forms the inner wall of the 

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

 of a screw, is a thiri lamina of bone, termed the lamina spiralis, 

 which partially subdivides the osseous tube into two passages. 



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

 spicuous than the others, called 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 acoustic nerve to the membranous 

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

 is the peripheral ganglion of the cochlear part of the acoustic 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. It 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 helicotrema. 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. 



It can now be understood how vibrations of the membrana tympani 

 are communicated to the perilymph within the osseous labyrinth. The 

 chain of auditory ossicles, through the base of the stapes, affects the 

 perilymph in the vestibule. The vibrations of the perilymph passing 

 along the scala vestibuli into the scala tympani affect in turn the secondary 

 membrane of the tympanum, which is stretched across the fenestra cochleae. 

 With every inward movement of the membrana tympani and of the base 

 of the stapes, there is an outward movement of the membrane of the 

 fenestra cochleae, and vice versa. The vibrations of the perilymph affect 

 the endolymph in the membranous labyrinth, and thus excite the termina- 

 tions of the acoustic nerve. 



Membranous Labyrinth. In the vestibule there are two mem- 

 branous sacs, termed the utricle and the saccule. The utricle occupies 

 the recessus ellipticus on the upper wall of the vestibule, and lies above 



