



THE INTERNAL EAR OR LABYRINTH 1047 



and the stylomastoid branch of the posterior auricular, which supplies the back part of ijbe 

 tympanic cavity and mastoid cells. The smaller arteries are the petrosal branch of the middle 

 meningeal, which enters through the hiatus of the facial canal; a branch from the ascending 

 pharyngeal, and another from the artery of the pterygoid canal, which accompany the auditory 

 tube; and the tympanic branch from the internal carotid, given off in the carotid canal and 

 perforating the thin anterior wall of the tympanic cavity. The veins terminate in the pterygoid 

 plexus and the superior petrosal sinus. The nerves constitute the tympanic plexus, which 

 ramifies upon the surface of the promontory. The plexus is formed by (1) the tympanic branch 

 of the glossopharyngeal; (2) the caroticotympanic nerves; (3) the smaller superficial petrosal 

 nerve; and (4) a branch which joins the greater superficial petrosal. 



The tympanic branch of the glossopharyngeal (Jacobson's nerve) enters the tympanic cavity 

 by an aperture in its floor close to the labyrinthic wall, and divides into branches which 

 ramify on the promontory and enter into the formation of the tympanic plexus. The superior 

 and inferior caroticotympanic nerves from the carotid plexus of the sympathetic pass through 

 the wall of the carotid canal, and join the branches of the tympanic branch of the glossopharyn- 

 geal. The branch to the greater superficial petrosal passes through an opening on the laby- 

 rinthic wall, in front of the fenestra vestibuli. The smaller superficial petrosal nerve, from 

 the otic ganglion, passes backward through a foramen in the middle fossa of the base of the 

 skull (sometimes through the foramen ovale), and enters the anterior surface of the petrous 

 part of the temporal bone through a small aperture, situated lateral to the hiatus of the facial 

 canal; it courses downward through the bone, past the genicular ganglion of the facial nerve, 

 receiving a connecting filament from it, and enters the tympanic cavity, where it communicates 

 with the tympanic branch of the glossopharyngeal, and assists in forming the tympanic plexus. 



The branches of distribution of the tympanic plexus are supplied to the mucous membrane 

 of the tympanic cavity; a branch passes to the fenestra vestibuli, another to the fenestra cochleae, 

 and a third to the auditory tube. The smaller superficial petrosal may be looked upon as the 

 continuation of the tympanic branch of the glossopharyngeal through 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 mandibular through the otic ganglion, and the Stapedius 

 by a branch from the facial. 



The chorda tympani nerve crosses the tympanic davity. It is given off from the sensory part 

 of the facial, about 6 mm. before the nerve emerges from the stylomastoid foramen. It runs 

 from below upward and forward in a canal, and enters the tympanic cavity through the iter 

 chordae posterius, and becomes invested with mucous membrane. It traverses the tympanic 

 cavity, crossing medial to the tympanic membrane and over the upper part of the manubrium 

 of the malleus to the carotid wall, where it emerges through the iter chordae anterius (canal 

 of Huguier). 



The Internal Ear or Labyrinth (Auris Interna). 



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 

 within the petrous part of the temporal bone, and the membranous labyrinth, a 

 series of communicating membranous sacs and ducts, contained within the bony 

 cavities. 



The Osseous Labyrinth (labyrinthus osseus] (Figs. 920, 921). 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 lined by 

 periosteum; they contain a clear fluid, the perilymph, in which the membranous 

 labyrinth is situated. 



The Vestibule (vestibulum) . The vestibule is the central part of the osseous 

 labyrinth, and is situated medial to the tympanic cavity, behind the cochlea, and 

 in front of the semicircular canals. It is somewhat ovoid in shape, but flattened 

 transversely; it measures about 5 mm. from before backward, the same from above 

 downward, and about 3 mm. across. In its lateral or tympanic wall is the fenestra 

 vestibuli, closed, in the fresh state, by the base of the stapes and annular ligament. 

 On its medial wall, at the forepart, is a small circular depression, the recessus 

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

 holes (macula cribrosa media) for the passage of filaments of the' acoustic nerve 

 to the saccule; and behind this depression is an oblique ridge, the crista vestibuli, 



