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 margin 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, 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 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 
