THE INTERNAL EAR. 921 



tympanum by an aperture in its floor, close to the inner wall, and divides into 

 branches which are contained in grooves upon the surface of the promontory 

 forming the tympanic plexus. 



Ir< branches of distribution are one to the fenestra rotunda, one to the fenestra 

 ovalis, and one to the lining membrane of the tympanum and Eustachian tube. 



Its branches of communication are three, and occupy separate grooves on the 

 surface of the promontory. One branch, the small deep petrosal, arches forward 

 and downward to the carotid canal to join the carotid plexus. A second, the long 

 petrvsiil //'///', runs forward through a canal close to or in the processus cochleari- 

 formis. and enters the foramen lacerum medium, where it joins the carotid plexus 

 of the sympathetic, and generally the large superficial petrosal nerve. The third 

 branch runs upward through the substance of the petrous portion of the temporal 

 bone. In its course it passes by the gangliform enlargement of the facial nerve, 

 and. receiving a connecting filament from it, becomes the small superficial petrosal 

 ni'i-i'i'. It then enters the skull through a small aperture, situated external to the 

 hiatus Fallopii on the anterior surface of the petrous bone, courses forward across 

 the base of the skull, and emerges through a foramen in the middle fossa (sometimes 

 through the foramen ovale) and joins the otic ganglion. 



The chorda tympani leaves the facial about a quarter of an inch above the 

 exit of the latter. It enters the tympanum through the iter chordce posterius, 

 and liecomes invested with mucous membrane. It passes forward, between the 

 handle of the malleus and vertical ramus of the incus, and leaves the tympanum 

 through the iter chordre anterius. 



The Internal Ear, or Labyrinth (Fig. 546). 



The internal ear has two main divisions, the osseous and membranous laby- 

 rinths. They are called labyrinths from the 



complexity of their shapes. The osseous laby- Sup. mof. 



rinth consists of three parts the vestibule, Aq - 



semicircular canals, and cochlea. It is formed 

 by a series of cavities channelled out of the 

 substance of the petrous bone, communicating ^ 

 externally with the cavity of the tympanum 



through the fenestrae ovalis and rotunda, and 



internally with the meatus auditorius internus, Prommt Fen - roL > 



bv means of minute bonv canals which contain 



J ,. , . , FIG. ->46. The bony labyrinth of the left 



the auditory nerve-filamentS. Within the OS- ear, seen from the outer side and somewhat 

 i i "' , i . j , i i from below ? (Gegenbaur). 



seous labyrinth is contained the membranous 



labyrinth, upon which the ramifications of the auditory nerve are distributed. 

 The Vestibule (Fig. 547) is the common central cavity of communication between 

 the parts of the internal ear. It is situated on the inner side of the tympanum, 

 behind the cochlea, and in front of the semicircular canals. It is somewhat ovoidal 

 in shape from before backward, flattened from within outward, and measures 

 about one-fifth of an inch from before backward, as well as from above downward, 

 being narrower from without inward. On its outer or tympanic wall is the 

 fenestra ovalis, closed, in the recent state, by the base of the stapes and its annular 

 ligament. On its inner wall, at the fore part, is a small circular depression, fovea 

 hemispherica, which is perforated, at its anterior and inferior part, by several 

 minute holes (macula cribrosd) for the passage of the filaments of the auditory 

 nerve ; and behind this depression is a vertical ridge, the pyramidal eminence 

 (crista vestibuli). At the hinder part of the inner wall is the orifice of the aque- 

 ductus vestibuli, which extends to the posterior surface of the petrous portion of 

 the temporal bone. It transmits a small vein, and contains a tubular prolonga- 

 tion (ductus cndolymphaticus) which, derived from the saccule and utricle, in a 

 manner to be described later, ends in a cul-de-sac. On the upper wall or roof is 

 a transverse oval depression, fovea semi-elliptica, separated from the fovea hem- 



