1548 A MANUAL OF ANATOMY 



Nerves of the Tympanum — i- Tympanic Plexus. — This plexus is situated 

 on the inner wall of the tympanic cavity, more particularly over the pro- 

 montory. It is formed by (a) the tympanic branch (Jacobson's nerve) of 

 the petrous ganglion of the glosso-pharyngeal, and (6) the carotico-tympanic 

 branch of the carotid plexus of the sympathetic. 



Branches. — These are distributed to the mucous membrane of the tym- 

 panum, Eustachian tube, and mastoid cells. Besides these, there are two 

 named branches, called the small superficial petrosal and the small deep 

 petrosal. The small superficial petrosal nerve is to be regarded as the con- 

 tinuation of Jacobson's nerve. After issuing from the tympanic plexus it 

 leaves the tympanum at the upper part of the anterior wall, and traverses 

 a small canal in the petrous part of the temporal bone, passing beneath the 

 upper portion of the canal which lodges the tensor tympani muscle. Whilst 

 in this canal it is joined by a small branch from the geniculate ganglion of 

 the facial nerve, and it emerges from the canal through the accessory hiatus 

 on the superior surface of the pars petrosa. It then passes forwards and 

 inwards beneath the dura mater, and leaves the cranial cavity through the 

 canaliculus innominatus, or through the fissure between the pars petrosa and 

 the great wing of the sphenoid, or through the foramen ovale. Having entered 

 the zygomatic fossa, it joins the otic ganglion, to which it conducts glosso- 

 pharjmgeal fibres. 



The small deep petrosal nerve passes forwards through a minute canal 

 within the processus cochleariformis to the upper aspect of the foramen 

 lacerum medium, where it joins the carotid plexus of the sympathetic and 

 the great deep petrosal, or, it may be, the great superficial petrosal, nerve. 

 In this manner some of its fibres pass in the Vidian nerve to Meckel's gang- 

 lion. 



2. Chorda Tympani Nerve. — This nerve arises from the facial nerve in the 

 lower portion of the descending part of the aqueduct of Fallopius. Having 

 ascended for a little in the aqueduct, it leaves it by passing forwards through 

 a minute canal, called the iter chordce posterius, which opens on the posterior 

 wall of the tympanum, below the level of the pyramid. The nerve, invested 

 by a fold of mucous membrane, now passes forwards internal to the mem- 

 brana tympani, and over the upper part of the handle of the malleus. It 

 then leaves the tympanic cavity through a passage situated at the inner end 

 of the fissure of Glaser, called the iter chordce anterius, or canal ol Huguier^ 

 after which it joins the lingual nerve. 



Internal Ear. 



The internal ear is the essential part of the organ of hearing, and 

 is known as the labyrinth from its remarkable complexity. It 

 consists of two parts, namely, the osseous labyrinth and the mem- 

 branous labyrinth. 



Osseous Labyrinth. 



The osseous labyrinth is a cavity situated within the petrous part 

 of the temporal bone, and it is divided into three parts, namely, 

 the vestibule, the semicircular canals, and the cochlea. These 

 divisions are lined with a delicate periosteum, between which and 

 the contained membranous labyrinth there is a clear fluid, called 

 the perilymph. 



Vestibule. — ^The vestibule is the central division of the osseous 

 labyrinth. The semicircular canals lie behind it, and the cochlea 

 is situated in front of it. The outer wall presents the fenestra ovalis, 

 which is occupied by the foot-piece of the stapes and its annular 



