INTERNAL EAR. 703 



Nerves to muscles. The Tensor Tympani is supplied by a branch from the 

 otic ganglion; the Laxator Tympani, and the Stapedius, by a filament from 

 the facial (Somm erring). 



The nerves distributed to the lining membrane are derived from the tympanic 

 plexus. 



Communications between the following nerves take place in the tympanum : 

 the tympanic branch from the petrous ganglion of the glosso-pharyngeal; a 

 filament from the carotid plexus; a branch which joins the great superficial 

 petrosal nerve from the Vidian; and a branch to the otic ganglion (small super- 

 ficial petrosal nerve). 



The tympanic branch of the glosso-pharyngeal (Jacobson's nerve) enters the 

 tympanum by an aperture in its floor, close to the inner wall, and ascends on 

 to the promontory. It distributes filaments to the lining membrane of the 

 tympanum, and divides into three branches, which are contained in grooves on 

 the promontory, and serve to connect this with other nerves. One branch runs 

 in a groove, forwards and downwards, to an aperture situated at the junction 

 of the anterior and inner walls, just above the floor, and enters the carotid 

 canal, to communicate with the carotid plexus of the sympathetic. The second 

 branch is contained in a groove which runs vertically upwards to an aperture 

 on the inner wall of the tympanum, just beneath the anterioi pyramid, and in 

 front of the fenestra ovalis. The canal leading from this opens into the hiatus 

 Fallopii, where the nerve contained in it joins the great petrosal nerve. The 

 third branch ascends towards the anterior surface of the petrous bone; it then 

 passes through a small aperture in the sphenoid and temporal bones to the 

 exterior of the skull, and joins the otic ganglion. As this nerve passes by the 

 gangliform enlargement of the facial, it has a connecting filament with it. 



The chorda tympani quits the facial near the stylo-mastoid foramen, enters 

 the tympanum at the base of the pyramid, and arches forwards across its cavity 

 between the handle of the malleus and long process of the incus. ; to an opening 

 internal to the fissura Glaseri. It is invested by a reflection wf the lining mem- 

 brane of tha tympanum. 



INTERNAL EAR, OR LABYRINTH, 



The Internal Bar is the essential part of the organ, receiving the ultimate 

 distribution of the auditory nerve. It is called the labyrinth, from the complexity 

 of its shape, and consists of three parts, the vestibule, 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 tympa- 

 num, through the fenestra ovalis and rotunda; and internally, with the meatus 

 auditorius internus, which contains the auditory nerve. Within the osseous 

 labyrinth is contained the membranous labyrinth, upon which the ramifications 

 of the auditory nerve are distributed. 



The Vestibule (Fig. 381) 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 backwards, flattened from side to side, 

 and measures about one-fifth of an inch from before backwards, as well as from 

 above downwards, being narrower from without inwards. On its outer, or tym- 

 panic ivall, 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 cribosa), for the passage of the 

 filaments of the auditory nerve; and behind this depression is a vertical ridge, 

 the pyramidal eminence. At the hinder part of the inner wall is the orifice of 

 the aquseductus vestibull, which extends to the posterior surface of the petrous 

 portion of the temporal bone. It transmits a small vein, and, according to 



