The Bones of the Skull. 



147 



m-so. 



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 basi- 

 occipital, but the swollen portion is separated from the latter by a 

 broad groove terminating posteriorly in the jugular fossa and the 

 jugular foramen. Immediately in front of the jugular fossa, the 

 rounded aperture of the external carotid foramen, transmitting 

 in the natural condition the internal carotid artery, leads into the 

 carotid canal of the interior 

 of tympanic portion. At the 

 anterior end of the groove, 

 communicating with the fora- 

 men lacerum, is the anterior 

 opening of the carotid canal, 

 the internal carotid foramen, 

 and on its lateral side the much 

 larger aperture of the auditory 

 (Eustachian) tube. The rela- 

 tions of these apertures are seen 

 to best advantage when the 

 petrotympanic 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 frorri 

 one foramen to the other. 



m-orc 



Fig. 64. Petrotympanic portion of the 

 auditory complex of tiie 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 tympani). 

 cm., mastoid cells; c.t., ' tympanic cavity; 

 i.e., 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. 



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 surface displayed being as indicated in Fig. 64. 

 The tympanum or middle ear is enclosed by the tympanic and 

 petromastoid portions of the temporal complex. The attached 



