18G 



ANATOMY OF THE RABBIT 



m-so. 



stylomastoid foramen lies between the latter and the external 

 acoustic meatus. 



The petrous portion, as viewed from its medial surface, is 

 roughly oblong; it is placed obliquely with reference to the basi- 

 occipital and basisphenoid. The parafloccular fossa occupies its 

 posterodorsal portion, and extends into the substance of the bone, 

 forming a much larger depression than is indicated by the diameter 

 of its rim. The related dorsal margin of the bone is occupied by a 

 groove which leads into a canal at its posterior margin. It indicates 

 the position of the lateral portion of the transverse sinus of the dura 

 mater. The ventral, thicker portion of the bone, enclosing the 

 apertures of the internal acoustic meatus and the facial canal, is 

 also that lodging the vestibulum and cochlea of the internal ear. 

 A small aperture at its antero ventral angle, visible only when the 

 petrotympanic is freed from its connections, represents the hiatus 

 canalis facialis of the human skull. It transmits the great super- 

 ficial petrosal nerve, a branch 

 of the facial nerve passing to 

 the sphenopalatine ganglion. 



The tympanic surface of the 

 petrous portion is described be- 

 low in connection with the struc- 

 tures of the tympanic cavity. 

 The tympanic portion forms 

 the spherical, expanded, shell - 

 like, tympanic bulla, which con- 

 tains in its interior the tympanic 

 cavity, and is continuous dorsally 

 with the bony enclosure of the 

 external acoustic meatus. The 

 boundary between the two is 

 indicated externally by a shallow 

 oblique groove, the position of 

 which indicates roughly that of 

 the tympanic membrane within. 

 The medioventral margin of the 

 bone is articulated with the basioccipital, but the swollen portion 

 is separated from the latter by a broad groove terminating 



7n-ae 



Fig. 90. Petrotympanic portion of the 

 auditory complex of the left side X 3. 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 tym- 

 pani); 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., supra- 

 occipital margin of petromastoid; p.m., 

 mastoid process; st., stapes; t.a., aperture of 

 auditory tube. 



