LABYRINTH. VESTIBULE. 513 



mastoid cells, which it lines throughout, and passes through the Eus- 

 tachian tube to become continuous with the mucous membrane of the 

 pharynx. 



Vessels and Nerves. The Arteries of the tympanum are derived 

 from the internal maxillary, internal carotid, and posterior auricular. 



Its Nerves are 1. Minute branches from the facial, which are 

 distributed to the stapedius muscle. 2. The chorda tympani, which 

 leaves the facial nerve near the stylo-mastoid foramen, and arches 

 upwards to enter the tympanum at the root of the p} r ramid ; it then 

 passes forwards between the handle of the malleus and long process of 

 the incus, to its proper opening in the fissura Glaseri. 3. The tym- 

 panic brandies of Jacobsoii's nerve, which are distributed to the mem- 

 branes of the fenestra ovalis and fenestra rotunda, and to the Eusta- 

 chian tube, and form a plexus by communicating with the carotid 

 plexus, otic ganglion, and Vidian nerve. 4. A filament from the otic 

 ganglion to the tensor tympani muscle. 



INTERNAL EAR. 



The Internal ear is called labyrinth, from the complexity of its com- 

 munications ; it consists of a membranous and an osseous portion. 

 The osseous labyrinth presents a series of cavities which are channeled 

 through the substance of the petrous bone, and is situated between the 

 cavity of the tympanum and the rneatus auditorius internus. It is 

 divisible into the 



Vestibule, 



Semicircular canals, 



Cochlea. 



The VESTIBULE is a small three-cornered cavity, compressed from 

 without inwards, and situated immediately within the inner wall of 

 the tympanum. The three corners which are named ventricles or 

 cornua are placed, one anteriorly, one superiorly, and one posteriorly. 



The Anterior ventricle receives the oval aperture of the scala vesti- 

 buli ; the superior, the ampullary openings of the superior and hori- 

 zontal semicircular canals ; the posterior ventricle receives the am- 

 pullary opening of the oblique semicircular canal, the common aperture 

 of the oblique and perpendicular canals, the termination of the hori- 

 zontal canal, and the aperture of the aquasductus vestibuli. In the 

 anterior ventricle is a small depression, which corresponds with the 

 posterior segment of the cul de sac of the meatus auditorius internus ; 

 it is called the fovea hemispherica, and is pierced by a cluster of small 

 openings, the macula cribrosa. In the superior ventricle of the vesti- 

 bule is another small depression, the fovea elliptica, which is separated 

 from the fovea hemispherica by a projecting crest, the eminentia 

 pyramidalis. The latter is pierced by numerous minute openings for 



2 L 



