SPECIAL ANATOMY. 163 



to wliicli tlie fibres of tlie nerve of that special sense 

 run, and a special arrangement whereby those influences 

 which cause the impressions are best able to alter the 

 condition of the cells. The ear is divided into external, 

 middle, and internal ears. The first has been already 

 noticed ; the internal ear is that part in which are situated 

 the cells of hearing. It is divided into cochlea, vestibule, 

 and semicircular canals. The vestibule is the central 

 cavity. It has seven openings into it, those from the 

 semicircular canals, one from the scala vestibuli of the 

 cochlea, and one from the middle ear, fenestra ovalis. It 

 contains a membranous sac, the membranous vestibule, 

 which is divided into two parts. The superior, utriculus, 

 receives the openings of the membranous semicircular 

 canals. It is separated by a membranous partition from 

 the inferior or sacculus, which has no openings. This 

 membranous portion is in contact both externally and in- 

 ternally with a serous fluid. That surrounding it is the 

 ]}erihjmpli, that contained inside of it is the enclolymph and 

 contains numerous minute calcareous particles, otoconites 

 or otolithes. The membranous semicircular canals ex- 

 actly resemble the bony cavities in which they are placed. 

 They contain endolymph and are surrounded by perilymph. 

 They are three in number, superior, internal, and p>osterior, 

 and each canal opens by its two extremities into the ves- 

 tibule, one of these extremities is dilated to form the 

 ampulla. The undilated extremities of the superior and 

 internal canals open by a common orifice into the vestibule. 

 To the ampullae run bi'anches of the eighth or auditory 

 nerve. 



The cochlea is a cavity hollowed out in the substance of the 

 j)etrouS temporal bone, shaped like a snail's shell, consisting 

 of a canal arranged in a spinal manner around a central bony 

 tube, modiolus. This canal is largest at its commence- 

 ment, and superiorly comes to a point. It is divided into two 

 parts by a spiral plate, which is internally attached to the 

 modiolus and projects into the canal. The division is com- 

 pleted by a membrane consisting of two layers between 

 Tvhich is the scala media of the cochlea. The two portions 

 of the canal, termed respectively superior or scala vestibuli, 

 inferior or scala tympani, communicate superiorly. The 

 former inferiorly terminates in the vestibule, the latter at 

 the fenestra rotunda, an opening in the bone closed by a 



