THE SENSES. 185 



resemblance to the key-board of a piano. In close relation with the rods 

 of Corti and the cells inside and outside them, and probably projecting 

 by free ends into the little "tunnel" containing fluid (roofed in by them), 

 are filaments of the auditory nerve. 



Membranous Labyrinth. This corresponds generally with the 

 form of the osseous labyrinth, so far as regards the vestibule and semi- 

 circular canals, but is separated from the walls of these parts by fluid, 

 except where the nerves enter into connection within it. As already men- 

 tioned, the membranous labyrinth contains a fluid called endolympli; and 

 between its outer surface and the inner surface of the walls of the vesti- 

 bule and semicircular canals is another collection of similar fluid, called 

 perilympli; so that all the sonorous vibrations impressing the auditory 

 nerves on these parts of the internal ear, are conducted through fluid to 

 a membrane suspended in and containing fluid. In the cochlea, the 

 membranous labyrinth completes the septum between the two scales and 

 encloses a spiral canal, previously mentioned, called canalis membranaceus 

 or canalis cochlece (Fig. 362). The fluid in the scales of the cochlea is con- 

 tinuous with the perilympli in the vestibule and semicircular canals, and 

 there is no fluid external to its lining membrane. The vestibular portion 

 of the membranous labyrinth cofnprises two, probably communicating 

 cavities, of which the larger and upper is named the utriculus; the lower, 

 the sacculus. They are lodged in depressions in the bony labyrinth 

 termed respectively "fovea hemielliptica" and "fovea hemispheric^." 

 Into the former open the orifices of the membranous semicircular canals; 

 into the latter the canalis cochlece. The membranous labyrinth of all 

 these parts is laminated, transparent, very vascular, and covered on the 

 inner surface with nucleated cells, of which those that line the ampullae 

 are prolonged into stiff hair-like processes; the same appearance, but to a 

 much less degree, being visible in the utricule and saccule. In the cavities 

 of the utriculus and sacculus are small masses of calcareous particles, 

 otoconia or otoliths; and the same, although in more minute quantities, are 

 to be found in the interior of some other parts of the membranous 

 labyrinth. 



Auditory Nerve. For the appropriate exposure of the filaments of 

 the auditory nerve to sonorous vibrations all the organs now described are 

 provided. It is characterized as a nerve of special sense by its softness 

 (whence it derived its name of portio mollis of the seventh pair) and by 

 the fineness of its component fibres. It enters the labyrinth of the ear 

 in two divisions; one for the vestibule and semicircular canals, and the 

 other for the cochlea. 



The branches for the vestibule spread out and radiate on the inner 

 surface of the membranous labyrinth : their exact termination is unknown. 

 Those for the semicircular canals pass into the ampullae, and form, within 

 each of them, a forked projection which corresponds with a septum in the 



