THE SKELETAL SYSTEM 17 



to these foramina, into a flat process which extends to- 

 ward 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 or- 

 bito-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 pre- 

 sphenoid^ and anterior part of the basisphenoid, leaves 

 a pair of long slits dorsal to the choana. Observe the po- 

 sition 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, poste- 

 riorly, and the orbital process of the frontal bone, ante- 

 riorly. The shape and size of the orbitosphenoid can be 

 readily observed from the inside of the cranial cavity. 



Squamosal Bone (Os Squamosum) 



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, 



