456 INTERNAL EAR. 



4. In the Anterior wall are the 



Eustachian tube, 



Fissura Glaseri, 



Opening for the laxator tympani, 



Apertura chordae (exit). 



The tympanum is lined by a vascular mucous membrane, which 

 invests the ossicula and chorda tympani, and forms the internal 

 layer of the membrani tympani. From the tympanum it is reflected 

 into the mastoid cells, which it lines throughout, and passes through 

 the Eustachian tube to become continuous with the mucous mem- 

 brane 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 to the stylo-mastoid foramen, and arches 

 upwards to enter 'the tympanum at the root of the pyramid ; 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 

 tympanic branches of Jacobson's nerve, which are distributed to the 

 membranes of the fenestra ovalis and fenestra rotunda, and to the 

 Eustachian tube, and form a plexus by communicating with the 

 carotid plexus and otic ganglion. 4. A filament from the otic gan- 

 glion to the tensor tympani muscle. 



INTERNAL EAR. 



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

 communications : it consists of a membranous and an osseous por- 

 tion. 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 meatus auditorius in- 

 terims. 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 poste- 

 riorly. 



The Anterior ventricle receives the oval aperture of the scala 

 vestibuli ; the superior, the ampullary openings of the superior and 

 horizontal semicircular canals ; the posterior ventricle receives the 

 ampullary opening of the oblique semicircular canal, the common 

 aperture of the oblique and perpendicular canals, the termination oi 



