THE BOXES OF THE SKULL 187 



posteriorly in the jugular fossa and the jugular foramen. Immedi- 

 ately 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 the tympanic portion. At the anterior end of the groove, commu- 

 nicating with the foramen 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 

 relations 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 from one foramen to the other. 



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. 90. The 

 tympanum or middle ear is enclosed by the tympanic and petro- 

 mastoid 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 promontory (promon- 

 torium). Above and behind the promontory, the tympanic cavity 

 is extended toward the mastoid portion of the bone 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 con- 

 dition it is closed by a thin membrane which separates the tympanic 

 cavity from the perilymphatic space containing the membranous 

 labyrinth. The second aperture, the vestibular fenestra (fenestra 



