142 ANATOMY OF THE RABBIT. 
by a vacuity. The internal-surface bears a longitudinal groove, 
lodging in the natural condition the median vermis of the cerebellum. 
It is crossed at its anterior end by a shallow transverse groove 
(sulcus transversus), which marks the position of the transverse 
sinus of the dura mater. 
2. THE POSTERIOR SPHENOID. 
The sphenoid bone, as identified from the human condition, 
is a complex of elements belonging to two segments, namely, the 
posterior sphenoid (os sphenoidale posterius) and the anterior 
sphenoid (os sphenoidale anterius). In the rabbit, as in mammals 
generally, these segments are separate throughout life. 
The posterior sphenoid comprises: (1) a median portion, the 
body, or basisphenoid; (2) paired dorsolateral expansions, the 
greater wings (alae magnae), or alisphenoids; and (3) paired 
ventral projections, the pterygoid processes. 
The basisphenoid continues the basis cranii forward from the 
basioccipital to the body of the anterior sphenoid. It is united 
with the latter by the intersphenoidal synchondrosis. Its 
surfaces correspond for the most part to those of the basioccipital. 
The ventral surface forms the chief part of the bony roof of the 
nasopharynx. It is perforated in its middle by a round aperture, 
the foramen cavernosum, which leads into the interior of the 
bone. The dorsal surface is occupied by the hypophyseal fossa 
and related structures, namely, the dorsum sellae and the pos- 
terior clinoid processes. On the lateral surface of the base of 
the posterior clinoid process a faint groove, the sulcus caroticus, 
marks the course of the internal carotid artery. The interior of the — 
bone contains a cavity of considerable size, the sphenoidal sinus 
(sinus sphenoidalis), which communicates both with the foramen 
cavernosum and the hypophyseal fossa. 
The alisphenoid extends at first laterad, but soon changes its 
direction so that its axis beomes dorsoventral. At the same time 
the bone is rotated in such a way that its surfaces tend to fall in a 
transverse plane. It is bounded anteriorly by the orbitosphenoid, 
dorsally by the squamosal, and posteriorly by the petrotympanic. 
The anterior margin of its root encloses with the basisphenoid, 
