THE INTERNAL EAR. 859 



it communicates with Jacobson's nerve, and assists in forming the tympanic 

 plexus. 



The branches of distribution of the tympanic plexus are distributed to the 

 mucous membrane of the tympanum ; one special branch passing to the fenestra 

 ovalis, another to the fenestra rotunda, and a third to the Eustachian tube. The 

 small superficial petrosal may be looked upon as a branch from the plexus to the 

 otic ganglion. 



In addition to the tympanic plexus there are the nerves supplying the muscles. 

 The Tensor tympani is supplied by a branch from the third division of the fifth 

 through the otic ganglion, and the Stapedius by the tympanic branch of the facial. 



The chorda tympani nerve crosses the tympanic cavity. It is given off from 

 the facial as it passes vertically downward at the back of the tympanum, about 

 a quarter of an inch before its exit from the stylo-mastoid foramen. It passes 

 from below upward and forward in a distinct canal, and enters the cavity of the 

 tympanum through an aperture, iter chordce posterius, already described (page 

 853), and becomes invested with mucous membrane. It passes forward, through 

 the cavity of the tympanum, crossing internal to the membrana tympani and over 

 the handle of the malleus to the anterior inferior angle of the tympanum, and 

 emerges from that cavity through the iter chordce anterius, or canal of ffuguier. 

 It is invested by the fold of mucous membrane already mentioned, and therefore 

 lies between the mucous and fibrous layers of the membrana tympani. 



The Internal Ear or Labyrinth. 



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

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

 complexity of its shape, and consists of two parts : the osseous labyrinth, a series 

 of cavities channelled out of the substance of the petrous bone, and the membran- 

 ous labyrinth, the latter being contained within the former. 



The Osseous Labyrinth. 



The osseous labyrinth consists of three parts : the vestibule, semicircular canals, 

 and cochlea. These are cavities hollowed out of the substance of the bone, and 





Opening of aqueduclus vestibuli. 

 Bristle passed through foramen rotundtim. 



Opening of aqueductus cochleae. 

 FIG. 460 The osseous labyrinth laid open. (Enlarged.) 



lined by periosteum ; they contain a clear fluid, perilymph, or liquor Cotimnii, in 

 which the membranous labyrinth is situated. . 



The Vestibule (Fig. 460) is the common central cavity of commune 



