1492 



HUMAN ANATOMY. 



'Y\v& posterior wall is separated from the mastoid process by the tympano-mas- 

 toid fissure. The auricular branch of the pneumogastric (Arnold's nerve) passes 

 through this fissure to the posterior wall of the canal. The coughing, sneezing, or 

 vomiting that sometimes follows irritation of the canal, as from cleaning the ear, or ex- 

 amining it with instruments, is said to be due to a reflex effect upon the pneumogastric 

 through this branch. The auriculo-temporal branch of the trigeminal nerve enters 

 into its supply, and may explain the earache in cancer of the tongue or disease of the 

 lower teeth. Between the posterior wall of the meatus and the mastoid cells is a 

 thin plate of bone one or two millimeters in thickness. The sigmoid portion of the 

 lateral sinus is usually about 12 mm. back of this wall, and the mastoid antrum 

 about 5 mm. posterior to its deeper portion. 



The superior wall, which is from 4-5 mm. in thickness, often contains air- 

 cells between two plates of compact bone. Pus may burrow through it from the 

 canal to the interior of the cranium. At the posterior superior angle of the canal are 

 a number of small openings for blood-vessels and some connective tissue fibres, 

 through or along which pus may find its way from the mastoid antrum to the under' 

 surface of the periosteum in the meatus. 



THE MIDDLE EAR. 



The middle ear includes three subdivisions : the tympanic cavity , the Eustachian 

 tube, and the mastoid cells. 



It is ah irregular air-chamber, beginning on the lateral wall of the naso-pharynx 

 with the Eustachian tube, which leads upward, backward and outward, for about 

 one inch and a half into the temporal bone. Opposite the external auditory canal, 

 it widens into the tympanic cavity and continues backward into the mastoid cells. 



The Tympanic Cavity. 



The tympanic cavity (cavum tyrapani), also called the tympanum, is an irregu- 

 lar space within the temporal bone, lying between the internal ear and the external 



Fig. 1251. 



Superior ligament 



Head of malleus 

 Tendon of tensor tympani 



Posterior semicircular canal' 

 Facial nerve 



Vestibule 



Internal auditorj- canal 

 Cochlea 



Articular surface for incus 



Epityrapanic space 



Lateral ligament 



Handle of malleus 

 External auditory canal 

 Tympanic membrane, cut 



Probe in Eustachian tube 



Promontory 

 Tympanic cavity 



Frontal section through right ear, viewed from behind. X 25^. 



auditory canal. It is lined with mucous membrane and contains, in addition to the 

 air which enters by way of the Eustachian tube, the chain of ear ossicles. Its short- 

 est diameter, that between the middle of the tympanic membrane and the wall of the 

 labyrinth, is about 2 mm. The antero-posterior diameter is about 12 mm., whilst 

 the distance from the roof (tegmen tympani) to the floor, the supero-inferior diam- 

 eter, is about 15 mm. 



