THE MIDDLE EAR. 



853 



malleus and the greater part of the incus. Its diameter, including the attic, 

 measures about 15 mm. vertically and transversely. From without inward it 

 measures about 6 mm. above and 4 mm. belew ; opposite the centre of the tympanic 

 membrane it is only about 2 mm. It is bounded externally by the membrana tym- 

 pani and meatus ; internally, by the outer surface of the internal ear ; and commu- 

 nicates, behind, with the mastoid antrum and through it 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 (paries tegmentalis) is broad, flattened, and formed of a thin plate of 

 bone (tegmen tympani), which separates the cranial and tympanic cavities. It is 

 prolonged backward so as to roof in the mastoid antrum ; it is also carried forward 

 to cover in the canal for the Tensor tympani muscle, 



The floor (paries jugular is) is narrow, and is separated by a thin plate of bone 

 (fundus tympani} from the jugular fossa. 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. This ring ot bone is incomplete at 

 its upper part, forming a notch (incisura Rivini). Close to it are three small aper- 

 tures ; the iter chordae posterius, the Glaserian fissure, and the iter chordae anterius. 



Chorda 



FIG. 458. View of inner wall of tympanum. (Enlarged.) 



The iter chordce posterius is in the angle of junction between the posterior and 

 external walls of the tympanum, immediately behind the membrana tympani and 

 on a level with the upper end of the handle of the malleus ; it leads into a minute 

 canal, which descends in front of the aquneductus Fallopii, and terminates in that 

 canal near the stylo-mastoid foramen. Through it the chorda tympani nerve enters 

 the tympanum. 



The Glaserian 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 and anterior ligament of the malleus, 

 and gives passage to the tympanic branch of the internal maxillary artery. 



The iter chordce anterius is seen at the inner end of the preceding fissure ; it 

 leads into a canal (canal of Huguier], which runs parallel with the Glaserian fissure. 

 Through it the chorda tympani nerve leaves the tympanum. 



The internal wall of the tympanum (paries labyrinihicd) (Fig. 458) is vertical 

 in direction, and looks directly outward. It presents for examination the follow- 

 ing parts : 



Fenestra ovalis. Promontory. 



Fenestra rotunda. Ridge of the aquaeductus Fallopii. 



