ADAPTATIONS OF THE SKELETON 65 



temporal bar (made up of jugal and quadrato-jugal), such a 

 contrast to the strong zygomatic arch in the horse where 

 the food is ground, or in the tiger where the food is cut by 

 the back teeth. The dehcacy of the infra-temporal bar of 

 the bird's skull is correlated with the fact that it has no very 

 strong muscles attached to it and that it is not required to 

 resist pressure. Since the maxilla is the chief tooth-bearing 

 bone in reptiles and mammals, we cannot be surprised that 

 it should be weakly developed in birds. 

 



Fio. 15. — Cat's skull for comparison with bird's. CO., one of the 

 two occipital condyles ; O.R., occipital crest for the insertion of muscles 

 from neck and lower jaw ; C.R., sagittal crest for the insertion of muscles 

 from the lower jaw ; pa., parietal ; fr., frontal ; o., orbit for eye ; N., nasal ; 

 PMX., premaxilla (small) ; MX., maxilla (large); j., jugal; z., zygomatic 

 arch, formed of the jugal and anterior process of the squamosal (sq.) ; 

 c, post-glenoid process just behind the glenoid fossa of the squamosal 

 in which the condyle of the lower jaw or mandible (mn.) works ; it 

 prevents backward slipping of the lower jaw ; E., ear-hole ; A., ascending 

 process of the lower jaw, to which strong muscles are attached ; t.b., 

 tympanic bulla protecting the drum or tympanum ; s., sectorial or 

 carnassial, a special cutting tooth; ca., canine teeth. 



(d) The importance of the eyes in birds is registered in 

 the relatively large orbits with a thin vertical partition 

 between them (the inter-orbital septum due mainly to 

 presphenoid and mesethmoid), and with the large orbits 

 may be correlated the restriction of the brain-containing 

 portion of the skull to the posterior region which is very 

 markedly broadened out. Thus the skull is built on a 



F 



