IV VERTEBRATA: AVES 



537 



In respect to the vertebral column, it only needs mention that the verte- 

 bra articulate with each other by a so-called saddle-joint, that (in living 

 birds) only a few caudal vertebra persist behind the pelvis, that these are 

 partially fused to a single bone, the pygostyle, which supports the tail 

 feathers, and that, corresponding to the well-developed neck, there are 

 many cervical vertebra, among them an atlas and an axis or epistropheus, all 

 except the last two fused with the corresponding cervical ribs. 



The skull (fig. 588) resembles that of the lizards in the presence of a sin- 

 gle occipital condyle, in the movable condition of the quadrate upon the cra- 

 nium, and in the presence of a slender columella, but an os transversum is 

 lacking. The cranium, as a result of the increase in size of the brain, is more 



Pa 



FIG. 588. Skull of young bustard (from Claus). Als, alisphenoid; Ang, angu- 

 lare; Art, articulare; D, dentary; Ef, mesethmoid; Fr, frontal; Jmx, premaxillary; J, 

 jugal; L, lacrimal; MX, maxillary; N, nasal; Ol, exoccipital; Os, supraoccipital; Pa, 

 parietal; Pal, palatine; Pt, pterygoid; Q, quadrate; Qj, quaclratojugal; Sm, interorbital 

 septu-n ; Spb, basi- and presphenoid. 



spacious; the bones of its walls fusing early so that the sutures between 

 them are obliterated. The occipital condyle is on the under surface, so 

 that the skull is carried at nearly right angles to the axis of the vertebral 

 column. Teeth are lacking in living birds, although they occurred in 

 some fossil forms. In their place are hard horny sheaths covering the 

 jaws which are frequently carried back on the outside into a softer cere 

 (fig. 584, WH). 



The cranium consists of four occipitals, a basisphenoid prolonged in front 

 by a rostrum (possibly a part of a parasphenoid, possibly presphenoid) ; above, 

 the parietals and f rentals; and on the sides prootics, alisphenoids and orbito- 

 sphenoids, while the broad squamosals also enter its wall. The large meseth- 

 moid lies in the interorbital septum; the nasal cavity is roofed by the nasals 

 and beside them are the lacrimals. The quadrate articulates with the squamo- 

 sal, and from it extend forward internally the pterygoid, palatine, and vomer, 

 externally a zygomatic arch of quadratojugal and jugal to the maxillaries and 

 premaxillaries. The maxillaries are hinged in the ethmoidal region, so that in 

 opening the mouth there is besides the depression of the lower jaw an upward 

 motion of the upper jaw. 



