THE LABYRINTH. 533 



and semicircular canals, but is separated from the walls of 

 these parts by fluid, except where the nerves enter into con- 

 nection within it. In the cochlea, the membranous labyrinth 

 completes the septum between the two scalce, and incloses a 

 separate spiral canal, the canalis membranacea. As already 

 mentioned, the membranous labyrinth contains a fluid called 

 endolyrnph ; and between its outer surface and the inner sur- 

 face of the walls of the vestibule and semicircular canals is 

 another collection of similar fluid called perilymph: 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. The fluid in 

 the scales of the cochlea is continuous with the perilymph in the 

 vestibule and semicircular canals, and there is no fluid exter- 

 nal to its lining membrane. 



The vestibular portion of the membranous labyrinth com- 

 prises two, probably communicating cavities, of which the 

 larger and upper is named the utriculus ; the lower, the saccu- 

 lus. Into the former open the orifices of the membranous 

 semicircular canals ; into the latter the canalis membranacea 

 of the cochlea. 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 

 utricle and saccule. In the cavities of the utriculus and saccu- 

 lus are small masses of calcareous particles, otoconia or otolithes; 

 and the same, although in more minute quantities, are to be 

 found in the interior of other parts of the membranous laby- 

 rinth. 



The auditory nerve, for the appropriate exposure of whose 

 filaments to sonorous vibrations all the organs now described 

 are provided, 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 coch- 

 lea. The branches for the vestibule spread out and radiate 

 on the inner surface of the membranous labyrinth : their exact 

 determination 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 interior of 

 the ampulla. The branches for the cochlea enter it through 

 orifices at the base of the modiolus, which they ascend, and 

 thence successively pass into canals in the osseous part of the 

 lamina spiralis. In the canals of this osseous part or zone, 



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