frontal 



prema 



semicircular ca 



(dashed outii 



orbitosphen 



postporietal 



alisphenoid 



supraoccipital 



squamosa 



pterygoid 



basioccipital tympanic 

 premaxilla 



sup 



squamosal 

 frontal 



eta I 



parietal 



plisphenoid 

 postporietal 



pterygoid 



palatine 



orbitosphenoid ^^^^| 

 jugol 

 squamosal 



alisphenoid 

 basispherioid 

 basioccipital 



exoccipital 



supraoccipital 



premaxilla 



dentory^ 



basisphenoid 



exoccipital 

 basioccipital 



supraoccipital 



Figure 3-16. Head skeleton of SO-mm foetal rabbit. A, lateral view; B, dorsal view with roof of 

 left side removed; C, ventral view of skull, the lower |ow removed; D, medial v,ew of r.ght half of head. 



by the anteromedial center. A similar pattern appears to 

 be followed by the rabbit (Figure 3-16). In man there are 

 two or three pairs of centers enclosing the optic nerve 

 foramen and extending medially to fuse at the midline. The 

 third pair of centers, when present, is that in the pila pre- 

 optica. The most medial pair is identified with the term 

 presphenoid; however, since there is never a separate bone 

 here in even young mammals (after weening), the term is 



superfluous. The "presphenoid" centers are best developed 

 in the rat (Figure 3-17). In some insectivores there is ap- 

 parently only one pair of centers for this bone, and this 

 center appears in the preoptic pillar. These centers may re- 

 main separated by the anterior part of the basisphenoid or 

 join across the midline. There is some dermal extension 

 of these centers in forming the definitive bone. 



The alisphenoids. orbitosphenoids, and pterygoids fuse 



58 • OSTEOLOGY AND THE MAMMALIAN HEAD SKELETON 



