184 ANATOMY OF THE RABBIT 



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 consider- 

 able portion of the anteroventral wall of the middle cranial fossa. 



The ethmoidal process extends forward from the optic foramen. 

 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. 



The Squamosal Bone 



The temporal bone, or temporal complex, as recognized from 

 the human condition, is an association of three elements — squamo- 

 sal, tympanic, and periotic — which in the human skull are coalesced 

 to form a single bone. It is 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 in the wall of the skull. 

 In the rabbit the original elements are not coalesced, but the 

 periotic and tympanic bones are so closely associated that it is 

 proper to describe them as forming a petrotympanic bone. 



The squamosal bone (os squamosum) is a rectangular plate, 

 forming part of the lateral wall of the cranium, and bearing the 

 posterior root of the zygomatic arch. It is articulated anteriorly 

 with the orbitosphenoid and with the orbital portion of the frontal, 

 dorsally with the frontal and parietal, posteriorly with the supra- 

 occipital and petrotympanic, and ventrally with the alisphenoid. 

 Its posterior margin bears a prominent, slightly decurved squa- 

 mosal process (processus squamosus). It lies on the lateral surface 

 of the petrotympanic immediately above the opening of the bony 

 external acoustic meatus. The posterior root of the zygomatic 



