464 THE BIRDS xvi. 19- 



landing are possible, for instance, rooks may make a roll and sideslip 

 to the ground. 



20. The skull in birds 



The arrangement of the parts of the bird's skull is similar to that of 

 archosaurian reptiles (Fig. 242). Individual bones can be recognized 

 in the young, but they mostly become united in the adult to form a 

 continuous thin-walled structure that encloses the brain and sense- 

 organs and supports the beak (Fig. 278). Most birds are microsmatic; 

 the nasal passages are simple and the turbinals reduced. There is 

 seldom a complete bony secondary palate, such as there is in mammals, 

 instead the internal nostril opens into the mouth relatively far forward. 

 The large size of the brain and reduction of its olfactory portions are 

 responsible for the rounded form of the top of the head, and there are 

 very large orbits at the sides, separated by an ossified septum. The 

 base of the skull is formed by a basioccipital behind, carrying a single 

 occipital condyle. There is a large basisphenoid, covered ventrally by 

 a pair of basitemporals, probably representing the parasphenoid, the 

 front part of which makes a 'basisphenoid rostrum', as in archosaurs. 



The jaws are characteristically slender and elongated; in the more 

 advanced birds they have a very special form of support. The upper 

 part of the front of the skull is composed of the enlarged premaxillae, 

 the nostrils lying very far back and the nasal bones being small. The 

 palatines are long and fused far forward with the maxilla, while they 

 articulate movably behind with the pterygoids and base of the skull. 

 The pterygoid is a slender rod, itself movably articulated with the 

 skull and with the quadrate, which is a triangular bone with clearly 

 separate otic and basal articular processes. The upper jaw is thus a 

 long thin bar composed of maxillae, quadrato-jugal, and jugal, and as 

 in many reptiles it is capable of considerable movement ('kinesis'). 

 It is raised when the lower end of the quadrate moves forwards. This 

 mechanism is particularly well developed in parrots, where the beak 

 is freely hinged on the skull. This type of palatal arrangement is known 

 as neognathous. In some birds, such as the flightless ratites, the 

 palatines are shorter, the vomer larger, and the pterygoids less mov- 

 able, a condition called palaeognathous (p. 514). The lower jaw, also 

 elongated, consists of the articular bone and four membrane bones. 



21. The jaws, beak, and feeding mechanisms 



There is a complete lower temporal bar, composed of jugal and 

 quadrato-jugal bones. The temporal region is hard to interpret, but 



