IV. VERTEBRATA: AVES. 607 



single tarso- metatarsus (A, c), which has below as many articular 

 surfaces as there are toes (since the fifth toe only appears in the 

 embryo, at most four, in some three or even two, d-d'"). At the 

 same time the tarsals disappear by fusion with adjacent parts. 

 Even in reptiles (C) a part of the tarsals unite with the bones of 

 the shank, and the remainder with the metatarsals; in the birds 

 the union is completed, the proximal series fusing with the lower 

 end of the tibia to form a tibio-tarsus, the distal with the metacar- 

 pus to form the tarso-metatarsus, in this way producing the inter- 

 tarsal joint so characteristic of birds. 



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

 the vertebrae articulate with each other by a so-called saddle-joint, 

 that (in living birds) only a few caudal vertebrae persist behind the 

 pelvis, that these are partially fused to a single bone, the pygo- 

 style, which supports the tail feathers, and that, corresponding to 

 the well-developed neck, there are many cervical vertebras, among 

 them an atlas and an axis, all except the last two fused with the 

 corresponding cervical ribs. 



The skull (fig. 637) resembles closely that of the lizards in the 

 presence of a single occipital condyle, in the movable condition 

 of the quadrate upon the cranium, and in the presence of a slender 

 columella. On the other hand an os transversum is lacking. The 

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

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



Pal 



FIG. 637. Skull of young bustartt. (From Glaus.) Als, alisphenoid ; Ang, angulare ; 

 Art, articulare; ), dentary ; Et, mesethmoid ; Fr, frontal ; Jmx, premaxillary ; 

 J, jugal ; L, lachrymal ; MX, maxillary ; JV, nasal ; 01, exoccipital ; Os, supra- 

 occipital ; P<j, parietal ; Pal, palatine: Pf, pterygoid: Q, quadrate; QJ, quadrato- 

 jugal ; 8m, interorbital septum ; Spb, basi- and presphenoid. 



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 



