392 NEUROLOGY, OR ANATOMY OF NERVOUS SYSTEM 



The tympanic arteries come from the internal maxillary, 

 the stylomastoid branch of the posterior auricular, the petrosal 

 branch of the middle meningeal, the Eustachian branch of 

 the ascending pharyngeal, and from the internal carotid. The 

 veins reach the internal jugular by means of the middle menin- 

 geal and pharyngeal veins. 



The nerves of the tympanum are the muscular, already men- 

 tioned; the nerves to the mucous membrane from the tympanic 

 plexus; the communicating, viz., between Jacobson's nerve, 

 the sympathetic, and branches of the geniculate ganglion of 

 the seventh; and the chorda tympani. 



Jacobson's nerve (tympanic branch of the ninth) enters 

 the tympanum in the floor and passes to the promontory. 

 It forms the tympanic plexus, from which are supplied the 

 fenestrse, Eustachian tube, and lining membrane, and sends 

 off two communicating branches one to the carotid plexus, 

 one to the great superficial petrosal. It then receives a fila- 

 ment from the geniculate ganglion of the facial, and proceeds 

 to join the otic ganglion as the lesser superficial petrosal nerve. 



The chorda tympani arises from the facial near the stylo- 

 mastoid foramen, enters at the base of the pyramid, crosses 

 the tympanum between the long process of incus and handle 

 of malleus, and runs through the iter chordae anterius to the 

 canal of Huguier. 



The internal ear is the essential part of the hearing apparatus, 

 since here the auditory nerve is distributed. It is contained 

 in a cavity in the petrous bone, and is made up of the osseous 

 labyrinth and of the membranous labyrinths. 



The osseous labyrinth contains the membranous labyrinth, 

 and is divided into three parts: the vestibule, semicircular 

 canals, and cochlea. It communicates in the dry state with 

 the tympanum by means of the fenestrse. Between the osseous 

 and membranous labyrinth is a space occupied by a clear 

 fluid, the perilymph, and within the membranous labyrinth 

 is the endolymph. 



The vestibule is the central cavity lying between the cochlea 

 in front and the semicircular canal behind, the tympanum 

 being external. Its outer or tympanic wall presents ihefenestra 

 ovalis. 



Its inner wall has in front a depression, the fovea hemispherica, 

 pierced by several minute holes for the auditory filaments, 



