574 . HANDBOOK OF PHYSIOLOGY. 



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 scalce, and 

 incloses a spiral canal, previously mentioned, called canalis membfana- 

 ceus or canalis cochlece (Fig. 387). The fluid in the scalw of the coch- 

 lea is continuous with the perilymph in the vestibule and semicircular 

 canals, and there is no fluid external to its lining membrane. The ves- 

 tibular portion of the membranous labyrinth comprises two, probably 

 communicating cavities, of which the larger and upper is named the 

 iitriculus; the lower, the sacculus. They are lodged in depressions in 

 the bony labyrinth termed respectively "fovea hemielliptica " and 

 "fovea hemispherical' Into the former open the orifices of the mem- 

 branous 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 i\\sutri- 

 cule 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 soft- 

 ness (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 un- 

 known. Those for the semicircular canals pass into the ampulla?, 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, the nerves are arranged 

 in a plexus, containing ganglion cells. Their ultimate termination is not 

 known with certainty; but some of them, without doubt, end in the 

 organ of Corti, probably in cells. 



