VISCERAL ANATOMY. 159 



9. Scuto-auricularis internus. 

 Origin. Beneath scutiform. 

 Insertion. Base of concha. 



Action. Antagonizes No. 3. Turns ear outward and back- 

 ward. 



10. Mastoido-auricularis. 



Origin. Margin of auditory meatus. 

 Insertion. Into base of concha. 

 f Action. Constricts meatus. 

 Skin. Thin, vascular, and covered with fine hairs. 



EXTERNAL AUDITORY CANAL. 



This is a short canal lodged in the petrous portion of the tem- 

 poral bone, commencing at the external meatus and ending at the 

 middle ear, being shut off from that cavity by the membrana 

 tympani. The canal is lined with thin skin which contains a 

 number of ceruminous glands which secrete cerumen or wax. 



MIDDLE EAR OR TYMPANUM. 



The tympanum is a cavity situated in the interior of the tem- 

 poral bone at the junction of its mastoid and petrous portions, 

 communicating with the pharynx by a canal lined with mucous 

 membrane, the Eustachian tube. It has an external wall formed 

 by the membrana tympani, an internal, shutting it off from the 

 internal ear, showing two openings, the jenestra ovalis and fenestra 

 rotunda, separated by an elevation, the promontory, and a circum- 

 ference, which shows the openings of many mastoid cells. It is 

 lined with mucous membrane continuous with that of the pharynx, 

 and covered with pavement epithelium. 



The Membrana Tympani shuts off the external meatus from 

 the tympanum, is concave externally, convex internally, set 

 strongly in a bony depression and adherent internally to the 

 handle of the malleus. It consists of three layers, a middle fibrous 

 and muscular, an external of skin, and an internal of mucous 

 membrane. 



The Fenestra Ovalis, lying in front of the promontory, is oval 

 in shape, closed by the base of the stapes, and communicates with 

 the vestibule. 



The Fenestra Rotunda communicates with the scala tympani 

 and is closed in by a membrane. 



