Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/31

Rh to these foramina, into a flat process which extends toward the base of the zygomatic arch. This process runs posteriorly to the tympanic bulla but does not fuse with it. The process joins dorsally the squamosal and frontal bones. It forms a part of the posterior wall of the orbito-temporal fossa, sending a process forward to the palatine bone and alveolar process of the maxillary. The failure of the alisphenoid bones to fuse with the presphenoid and anterior part of the basisphenoid, leaves a pair of long slits dorsal to the choana. Observe the position of the anterior lacerated foramen.

The three fused constituents of the anterior sphenoid bone are the single median presphenoid and the two lateral orbitosphenoids. The presphenoid bone extends anteriorly from the basisphenoid. In the cerebral fossa its smooth surface decreases in width anteriorly until it is reduced to a thin septum between the two optic foramina. The ventral surface bears a keel which appears as a low ridge on the roof of the naso-pharyngeal duct. Each optic foramen is bounded medially by the presphenoid, laterally by the wedge shaped orbitosphenoid bone. The base of the latter is fused with the presphenoid. Its apex is thrust outward between the alisphenoid bone, posteriorly, and the orbital process of the frontal bone, anteriorly. The shape and size of the orbitosphenoid can be readily observed from the inside of the cranial cavity.

The squamosal bone resembles a very short-handled claw hammer if viewed from the lateral surface of the skull. The "handle" widens posteriorly, and with a spur from the occipital bone forms the posttympanic hook. The "handle" is bordered dorsaUy by the parietal bone,