90 Anatomy op the Rabbit. 



tympanic is disarticulated from the associated posterior sphenoid bone. 

 The auditory tube is then seen to lead directly into the tympanic cavity. 

 A fine bristle may be passed through the carotid canal from one foramen 

 to the other. 



6. THE STRUCTURES OF THE TYMPANIC CAVITY. 



The relations of the tympanic cavity and associated structures may 

 be studied with advantage in a skull from which the lateral wall of the 

 tympanic bulla and external acoustic meatus has been removed, the sur- 

 face displayed being as indicated in Fig. 35. The tympanum or middle 

 ear is enclosed by the tympanic and petromastoid portions of the temporal 

 complex. The attached margin of the tympanic bulla encloses a roughly 

 triangular area, into the ventral part of which the petrous portion of 

 the petromastoid projects as a smooth, white, convex ridge, the prom- 

 ontory (promontorium) . Above and behind the promontory the 

 tympanic cavity is extended toward the mastoid portion of the bone 



Fig. 35. Petrotympanic portion of the auditory complex of the left side x3. 

 The lateral portions of the tympanic bulla and external acoustic meatus have been 

 removed, exposing the structures of the tympanic cavity. MS, mastoid portion ; 

 P, petrous portion; T, tympanic portion (bulla tympani). 



cm., mastoid cells; c.t., tympanic cavity; f.c, cochlear fenestra; in., incus; -- .. 

 m.a.e., external acoustic meatus; m.m., manubrium of the malleus; m.so., ''■ 

 supraoccipital margin of petromastoid; p.m., mastoid process; St., stapes; t.a., * . 

 aperture of auditory tube. 



as the tympanic or mastoid antrum (antrum tympanicum), and the 

 interior of the mastoid portion is partly occupied by small extensions 

 of the tympanic antrum, termed the mastoid cells (cellulae mastoideae) . 

 At the anteroventral angle of the area already described, a deep notch 

 indicates the point of entrance of the auditory tube. The exposed 

 surface of the petromastoid presents two apertures, one of which, 

 situated posteroventrally, is open in the dried skull, and is the cochlear 

 fenestra (fenestra cochleae). In the natural condition it is closed by a 

 thin membrane which separates the tympanic cavity from the perilym- 

 phatic space containing the membranous labyrinth. The second 

 aperture, the vestibular fenestra (fenestra vestibuli), lies above and in 

 front of that just described. It is closed by the base of the stapes. 



The auditory ossicles (ossicula auditus) comprise three elements, 

 namely, the malleus, incus, and stapes, which bridge the space inter- 



