VERTEBRAL COLUMN 



59 



The greatest number of vertebrae is seen in Snakes, in which 

 there may be over 400. 



Birds. The vertebral column of Birds has many points of 

 resemblance with that of Reptiles both phylogenetically and onto- 

 genetically. In both groups the notochord usually eventually 

 disappears entirely, and the whole skeleton becomes strongly 

 ossified. Archaeopteryx, as well as the Cretaceous Ichthyornis, 

 possessed biconcave vertebrae, but in existing adult Birds this 

 character never occurs except in the free caudal vertebrae. 

 Cervical, thoracic, lumbar, sacral, and caudal regions can be distin- 

 guished. The arches always become united into a single mass 

 with the corresponding centra, not remaining separated from them 

 by sutures, as is the case in certain Reptiles : even the ligament 

 which keeps the odontoid process of the axis in its place may 



FIG. 47, 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 ; Po, odontoid process ; Ps, neural spine of 



axis ; Pt, transverse process ; W K, centrum of axis, and Sa, its saddle- 

 shaped articular surface for the third vertebra ; t, condylar facet. 



B, Ft, vertebrarterial foramen ; Ob, neural arch ; Pa, articular process ; Pt, Pt, 



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

 the cervical rib (R) ; Psi, median inferior process (hypapophysis). 



become ossified. Fibro-cartilaginous discs or menisci, perforated 

 in the centre by a ligament, 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 (heterocoslous) synovial articulations ; the upper part 

 of each bifurcated transverse process arises from the arch, the 

 lower from the centrum, and these may unite with the correspond- 

 ing forked rib, the vertebral artery and vein extending through 

 the foramen thus formed (Fig. 47). The ring-like atlas, with its 

 single facet for the occipital condyle, is relatively small, and the 

 odontoid is fused with the axis. In the thoracic and lumbar 

 regions more or fewer of the vertebrae usually become immovably 

 united together. 



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

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



