146 Anatomy of the Rabbit. 



cranial fossa. The dorsal portion of the bone corresponds in 

 thickness with the wing 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 niargin of the supraoccipital wing. 

 This portion is distinguishable by its pitted character. It forms 

 the mastoid portion (pars mastoidea) as distinguished 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 stylomastoid foramen lies between the latter and the ex- 

 ternal acoustic meatus. 



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

 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 anteroventral angle, only visible 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 below 

 in connection with the structures of the tympanic cavity. 



The tympanic portion forms the spherical, expanded, shell- 

 like, tympanic bulla, which contains in its interior the tympanic 

 cavity, and is continuous dorsally with the bony enclosure of the 



