48 COMPARATIVE ANATOMY 



them throughout life by sutures, as is the case in certain Reptiles : 

 even the ligament which keeps the odontoid process of the axis 

 in its place may become ossified. Fibro-cartilaginous discs or 

 menisci are present between the centra. In the cervical region, 

 which is extremely flexible and often very long, the centra are 

 in nearly all cases connected by means of saddle-shaped synomal 

 articulations ; the upper part of the bifurcated transverse processes 

 arises from the arch, the lower from the centrum, and these may 

 unite with the corresponding forked rib, the vertebral artery and 

 vein extending through the foramen thus formed (Fig. 36). 



In the thoracic and lumbar regions more or fewer of the 

 vertebras usually become immovably united together. 



The sacral region in Bird-embryos, like that in existing adult 

 Reptiles, consists of two vertebras only, the transverse processes of 



-Pa 



FIG. 36, A. ATLAS AND Axis (from the left side) ; and B, THIRD CERVICAL 

 VERTEBRA (ANTERIOR FACE) OF WOODPECKER (Picus viridis). 



A. Ob, A, arch and centrum of atlas ; f, condylar facet ; Po, odontoid process ; 



WK, centrum of axis, and Sa, its saddle-shaped articular surface for the 

 third vertebra ; Ps, neural spine of axis ; Pt, transverse process. 



B. Sa, articular surface of centrum ; Ob, upper arch ; Pa, articular process ; Pt, Pf, 



the two bars of the transverse process, shown on one side anchylosed with 

 the cervical rib (R) ; Ft, vertebrarterial foramen ; Psi, median inferior pro- 

 cess (hyparpophysis). 



which ossify separately and correspond to fused ribs, as in 

 Amphibians and Reptiles. During further development, however, 

 a number of other (secondary sacral) vertebras (thoracic, lumbar, 

 and caudal), with their rudimentary ribs, become fused with the 

 two primary ones (Fig. 37), so that the entire number of vertebras 

 in the sacrum may be as many as twenty-three. In Archasopteryx 

 the sacrum was much shorter than in existing Birds, and fewer 

 vertebras were united with it. 



In existing Birds the caudal region always exhibits a more or 

 less rudimentary character, and in its posterior portion the ver- 

 tebras usually fuse together to form a flattened bone, the pygostyle, 

 which supports the tail quills (Fig. 111). An exception to this rule 

 is found only amongst the Ratitas, in which all the caudal vertebras 



