170 VERTEBRATE SKELETON 



changed the position of the cranial base to an oblique plane so that 

 condyle and foramen magnum look obliquely downwards, and the 

 skull is usually carried at nearly a right angle to the axis of the body. 

 Less prominent is the absence of several bones, common in reptiles, 

 among them transversum, postfrontal, postorbital and epipterygoid. 

 It is not certain that none of these appear in ontogeny, fusing later 

 with other elements. 



The cranial bones fuse early, except in Ratites, with almost com- 

 plete obliteration of most sutures, especially in the brain case. 

 Frequently a suture persists between frontals and nasals which 

 allows motion of the tip of the upper jaw (fig. 183). The strepto- 

 stylic quadrate is very mobile and the arrangement of skeletal parts 

 is such that depression of the lower jaw forces the distal end of the 



Fig. 183. — Diagram of movement of upper jaw of birds (Boas). /, palatine; t, pter>% 



goid; s, zygomatic bar. 



quadrate forwards and this motion is transmitted forwards, both 

 by the zygomatic arch and by the palatine-pterygoid series (which 

 slide on the rostrum), to the upper jaw in front of the hinge between 

 nasals on one side, and frontals and mesethmoid on the other. In 

 this way the anterior part of the beak is elevated, increasing the gape. 

 This mobility of the upper jaw is especially well developed in parrots, 

 owls and goatsuckers. 



All modern birds lack teeth, but their germs are known in some 

 embryos. Some extinct birds had them well developed. Modern 

 birds have the beak enclosed in horny sheaths, variously developed 

 and largely supported on premaxillae and dentalia. The orbits are 

 very large, those of the two sides being separated by a very thin 

 interorbital septum, variously chondrified or ossified, or reduced to 

 membrane. Except in parrots, the orbit is continuous with the 

 single temporal fossa, this bounded laterally by the lower arcade of 

 diapsid reptiles which is always present. The orbits, except in 

 owls, look laterally, and the lower border is complete except in 

 parrots. Sclerotic bones are common. 



