144 ANATOMY OF THE RABBIT. 
of the bone is seen to be exposed to the cranial cavity only in its 
posterior portion, where it is occupied by the suleus chiasmatis 
and the optic foramina. That part of the floor immediately in 
front of the optic foramina is formed by the coalesced roots of the 
orbitosphenoids, the dorsal surface of the presphenoid being thus 
excluded. 
The orbitosphenoid forms a long, low plate, lying in the 
ventral portion of the orbit, and divided by a shallow notch at the 
level of the optic foramen into a posterior portion, the orbito- 
sphenoid proper, and an anterior portion, the ethmoidal process 
(processus ethmoidalis). The orbitosphenoid proper lies behind 
the optic foramen. It is in contact dorsally with the orbital portion 
of the frontal, and ventrally with the alisphenoid; it assists the 
latter in the formation of the superior orbital fissure. Its posterior 
tip is in contact with the squamosal. Its internal surface forms a 
considerable portion of the anteroventral wall of the middle cranial 
fossa. 
The ethmoidal process extends forward from the optic fora- 
men. Its dorsal margin is articulated with the orbital portion of 
the frontal, and its ventral margin with the orbital portion of the 
palatine. Anteriorly it projects toward the lacrimal bone, thus 
occupying, in part, a space which, in the typical mammalian skull, 
is filled by the lamina papyracea of the ethmoid. Its internal 
surface is associated with the ethmoid bone and with the nasal 
cavity. It falls for the most part below the level of the cranial 
cavity. 
4. THE SQUAMOSAL BONE. 
The temporal bone, or temporal complex, as recognized 
from the human condition, is an association of three elements— 
squamosal, tympanic, and periotic—which in the human skull 
are coalesced to form a single bone. It ‘s usually described as 
consisting of four portions, of which the squamosal and tympanic 
portions are. two, while the periotic bone is considered to consist 
of two others, one of which, the petrous portion, is a solid white 
portion lodging the internal ear, while the second, or mastoid 
portion, is a mass of less compact character appearing externally 
