Sauropsida: Class Ave6 517 



Fig. 406. Diagram illustrating the movement of the upper jaw of birds. (1) 

 Palatine; (q) quadrate; (t) pterygoid; (z) zygomatic arcade. (After Boas. Courtesy, 

 Kingsley: "Comparative Anatomy of Vertebrates," Philadelphia, The Blakiston 

 Company.) 



brain. Immediately in front of the brain-case, the enormous orbits 

 (the right and left concavities which contain the eyeballs) occupy 

 nearly the entire width of this narrower region, being separated only 

 by a thin median vertical plate of bone (or partly cartilage), the 

 interorbital septum (Fig. 405A). This is in striking contrast to the 

 relations of the corresponding parts in amphibians and early reptiles, 

 in which the comparatively small orbits are far apart and the longer, 

 narrower brain extends forward between the eyes. The form of skull in 

 this case is called platybasic — a type found in elasmobranchs, some 

 ganoids, lungfishes, amphibians, and early reptiles. In modern reptiles 

 the orbits, more or less enlarged, approach one another and in many 

 cases, most notably lizards, are separated only by a thin interorbital 

 septum as in birds — a type of skull called tropibasic. In birds the 

 tropibasic form is carried to its extreme. It is as if the eyes, enlarging 

 in a narrowing region of the skull, had crowded the brain backward, 

 forcing it to atone for shortness by adding width and height. 



The quadrate bone in birds is movably joined to the otic region, 

 as in Squamata (Figs. 405, 406). Also, the other bones of the upper jaw 

 are capable of more or less movement relative to the cranium, some- 

 what as in snakes — a condition most marked in parrots which can 

 swing the upper beak up and down through a large angle. 



A most striking feature of the skull is the extensive fusion of the 

 bones in the adult. In most cases the sutures become nearly or quite 

 obliterated, so that recognition of the bony elements which make up 

 the skull is possible only during the period of growth while the sutures 

 are still open. The bony tissue of the skull, too, is very highly calcified 

 and therefore especially hard. And most of the bones are exceedingly 

 thin. These peculiarities combine to produce a skull of maximum 

 strength with minimum weight. A further saving in the weight of the 

 head results from absence of teeth. The earlier birds known from 

 fossils had teeth, but no modern adult bird has teeth. In the jaws of 

 the embryo appear temporary configurations of cells which resemble 

 early stages in development of teeth. The bird's jaws are ensheathed 



