PHYLUM VIII 



VKRTEBRATA 



257 



caudal regions, but, as will presently be explained, the sacrum or synsaci uni 

 of birds is a complex mass of vertebrae not comparable as a whole with the 

 sacrum of other vertebrates. The vertebrae are remarkable for their peculiar 

 saddle-shaped articulations, which allow great freedom of movement. In the 



A 



is 



-.'-\ 



Fig. 36-2. 



Hesperomis regalis, Marsh. Upper Cretaceous ; Kansas. Anterior (.4), and posterior (7;) aspect of thirteenth 

 cervical vertebra, Vi- d, Transverse process ; /, Costal canal for vertebral artery; nc, Neural canal : p. Para- 

 pophysis ; s, Rudimentary neural spine ; z, z', Anterior and posterior zygapophyses (after Marsh). 



Mesozoic Archaeopteryx and Ichihyomis the centra are slightly amphicoelous, as 

 is also the case in modern embryonic birds and in some of the caudal vertebrae 

 of adults. Opisthocoelous vertebrae occur among the dorsals of penguins and 

 in a few other birds, such as cormorants and gulls, but the atlas is the only 

 procoelous vertebra in the backbone of birds. 



The cervical region consists of from thirteen to twenty-five vertebrae, the 

 commonest number being fourteen or fifteen. The centra are mostly elongate 

 and freely movable upon one another, 

 save that in hornbills the atlas and axis 

 may unite, and in a few other birds, the 

 tinamous for example, the last cervical 

 may fuse with the dorsals. The dorsal 

 vertebrae number from six to ten, the 

 first of the series being always the first 

 vertebra connected with the sternum by 

 a dorsal and sternal rib. The dorsal 

 series includes also those rib - bearing 

 vertebrae which are united with the 

 sacrum, and the number of free dorsals 

 is much less, being as few as three in Petroica, only seven in the long-bodied 

 grebe, and eight in the crested auklet. The centra of these vertebrae are 

 comparatively short. 



In water birds (e.g. penguins and some auks) all the presacral vertebrae 

 may be free though their motion is restricted, but usually from two to four of 

 the dorsals are fused to stiffen the trunk for flight, one free vertebra being left 

 between these and the synsacrum. The latter is a very complex bone, usually 

 composed in the adult of dorsal, lumbar, sacral, and caudal vertebrae united 

 in one solid mass, as many as twenty vertebrae taking part in its formation. 

 The true sacrals are those two lying behind the cavity containing the kidneys, 

 having transverse processes and sacral ribs, reaching from their centra to the 

 ilia. The rib-like nature of these may be seen in embryos, particularly of the 



VOL. II S 



Fig. 363. 



Hesperomis regalis, .Marsh. Upper Cretaceous; 



Kansas. Lateral (.1). and anterior (7.'). aspects of 

 dorsal vertebra, 1/2 (after Marsh). 



