The Bones oe the Skull. 89 



cranial fossa. The dorsal portion of the bone corresponds in thickness 

 with the wins* of the supraoccipital with which it is articulated. The 

 posterior surface is applied to the anterior surface of the exoccipital, 

 and is thus excluded both from the cranial cavity and from the external 

 surface of the skull. Viewing the skull from behind, however, it is seen 

 that a small dorsal portion protrudes in a triangular space formed by 

 the dorsolateral margin of the exoccipital and the ventrolateral margin 

 of the supraoccipital wing. This portion is distinguishable by its pitted 

 character. It forms the mastoid portion (pars mastoidea) as distin- 

 guished from the solid white petrous portion (pars petrosa), which is 

 exposed to the cranial cavity, and which contains the structures of the 

 internal ear. The mastoid portion lies for the most part above the 

 tympanic cavity, but it is also continued ventrad between the external 

 acoustic meatus and the exoccipital as the mastoid process. The stylo- 

 mastoid 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 basioccipital and 

 basisphenoid. The fioccular 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 anteroventral angle, 

 only visible when the petrotympanic is freed from its connections, repre- 

 sents the hiatus canalis facialis of the human skull. It transmits the 

 great superficial petrosal nerve, a branch of the facial nerve passing to 

 the sphenopalatine ganglion. 



The tympanic surface of the petrous portion is described below in 

 connection with the structures of the tympanic cavity. 



The tympanic portion forms the spherical, expanded, shell-like, tym- 

 panic bulla, which contains 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 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 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 rela- 

 tions of these apertures are seen to best advantage when the petro- 



