112 8 THE ORGANS OF SPECIAL SENSE 



(Fig. S35), close to which are three small apertures the iter chordae posterius r 

 the Glaserian fissure, and the iter chordae anterius. 



The iter chordae posterius (canaliculus chordae tympani) (Fig. 796) 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 facial 

 canal and terminates in the aqueduct near the stylomastoid foramen. Through 

 it the chorda tympani nerve enters the tympanum. 



The Glaserian or petrotympanic fissure (fissura petrotympanica) [Glaseri]). (Fig. 

 838) 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 2 mm. 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 chordae anterius or canal of Huguier (Fig. 838) is seen at the inner end 

 of the preceding fissure; through it the|Sb*)rda tympani nerve leaves the tympanum. 



The inner wall of the tympanum Cflpries labyrinthica) (Figs. 835 and 839) is 

 adjacent to the labyrinth; it is vertical in direction, and looks directly outward. 

 It presents for examination the following parts: 



Fenestra ovalis. U~ **** * Promontory. 



Fenestra rotunda. wc-tdv^vi' Ridge of the facial canal. 



Prominence of the lateral semicircular canal. 



The fenestra ovalis (fenestra vestibuli) (Fig. 835) is a reniform opening leading 

 from the tympanum into the vestibule of the internal ear. Its long diameter 

 is directed horizontally, and its convex border is upward. In the recent state 

 it is occupied by the base of the stapes (Figs. 834 and 840), the circumference of 

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



The fenestra rotunda (fenestra cochleae] (Fig. 835) is situated below and a little 

 behind the fenestra ovalis, from which it is separated by a rounded elevation, the 

 promontory; at its border is a narrow ridge of bone (crista fenestrae cochleae}. 

 The fenestra rotunda is closed in the recent state by a membrane (membrana 

 tympani secundaria); 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, from the lining membrane 

 of the cochlea; and an intermediate or fibrous layer. 



The promontory (promo ntorium) (Fig. 839) is a rounded hollow prominence, 

 formed by the projection outward of the first turn of the cochlea; it is placed 

 between the fenestras, and is furrowed on its surface (sulcus promontorii) .for the 

 lodgement of the tympanic plexus. A minute spicule of bone frequently connects 

 the promontory to the pyramid. 



The prominentia canalis facialis (Fig. 839) indicates the position of the bony 

 canal in which the facial nerve is contained; this canal traverses the inner wall 

 of the tympanum above the fenestra ovalis, and behind that opening curves 

 nearly vertically downward along the posterior wall. 



The posterior wall of the tympanum (paries mastoidea) (Fig. 839) is wider 

 above than below, and presents for examination the 



Opening of the antrum. Fossa incudis. 



Pyramid. 



The opening of the antrum is a large irregular aperture, which extends back- 

 ward from the epitympanic recess and leads into a considerable air space, the 

 mastoid antrum (antrum tympanicum) (see p. 83). The antrnm communicates 



