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 sulcus 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. 1 1 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 craiiial 

 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 mamrnalian 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 hurhan 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 



