916 THE ORGANS OF SENSE. 



Middle Ear, or Tympanum. 



The middle ear, or tympanum, is an irregular cavity, compressed from without 

 inward, and situated within the petrous bone. It is placed above the jugular fossa ; 

 the carotid canal lying in front, the mastoid cells behind, the meatus auditorius 

 externally, and the labyrinth internally. It is filled with air, and communicates Avith 

 the pharynx by the Eustachian tube. The tympanum is traversed by a chain of 

 movable bones, which connect the membrana tympani with the labyrinth, and serve 

 to convey the vibrations communicated to the membrana tympani across the cavity 

 of the tympanum to the internal ear. 



The cavity of the tympanum measures about five lines from before backward, 

 three lines in the vertical direction, and between two and three in the transverse, 

 being a little broader behind and above than it is below and in front. It is bounded 

 externally by the membrana tympani and meatus, internally, by the outer surface 

 of the internal ear, and communicates, behind, with the mastoid cells, and in front 

 with the Eustachian tube and canal for the Tensor tympani. "Its roof and floor are 

 formed by thin osseous laminae, the one forming the roof being a thin plate situated 

 on the anterior surface of the petrous portion of the temporal bone, close to its 

 angle of junction with the squamous portion of the same bone. 



The roof is broad, flattened, and formed of a thin plate of bone which separates 

 the cranial and tympanic cavities. 



The floor is narrow, and corresponds to the jugular fossa, which lies beneath, 

 It presents, near the inner wall, a small aperture for the passage of Jacobson's 

 nerve. 



The outer wall is formed mainly by the membrana tympani, partly by the ring 

 of bone into which this membrane is inserted. Close to it are three small apertures 

 the iter chordae posterius, the Glaserian fissure, and the her chordae anterius. 



The aperture of the iter chordce posterius is in the angle of junction between the 

 posterior and external Avails of the tympanum, immediately behind the membrana 

 tympani and on a level Avith its centre ; it leads into a minute canal, Avhich descends 

 in front of the aqueductus Fallopii and terminates in that canal near the stylo- 

 mas toid foramen. Through it the chorda tympani nerve enters the tympanum. 



The Grlaserian fissure opens just above and in front of the ring of bone into 

 which the membrana tympani is inserted; in this situation it is a mere slit about 

 a line in length. It lodges the long process of the malleus and gives passage to 

 some tympanic vessels. 



The aperture of the iter chordce anterius is seen just above the preceding fissure ; 

 it leads into a canal (canal of Huguier}, Avhich runs parallel Avith the Glaserian 

 fissure. Through it the chorda tympani nerve leaves the tympanum. 



The internal wall of the tympanum (Fig. 542) is vertical in direction and looks 

 directly outAvard. It presents for examination the folloAving parts : 



Fenestra ovalis. Ridge of the aqueductus Fallopii. 



Fenestra rotunda. Promontory. 



The fenestra ovalis is a reniform opening leading from the tympanum into 

 the vestibule ; its long diameter is directed horizontally, and its convex border is 

 upward. The opening in the recent state is occupied by the base of the stapes, 

 which is connected to the margin of the foramen by an annular ligament. 



The fenestra rotunda is an aperture placed at the bottom of a funnel-shaped 

 depression leading into the cochlea. It is situated beloAv and rather behind the 

 fenestra ovalis, from Avhich it is separated by a rounded elevation, the promontory; 

 it is closed in the recent state by a membrane (membrana tympani secundaria, 

 Scarpa). This membrane is concave toward the tympanum, convex toward the 

 cochlea. It consists of three layers : the external, or mucous, derived from the 

 mucous lining of the tympanum ; the internal, or serous, from the lining membrane 

 of the cochlea ; and an intermediate, or fibrous layer. 



The promontory is a rounded hollow prominence, formed by the projection 

 outward of the first turn of the cochlea ; it is placed between the fenestrae, and 



