XII 



PHYLUM CHORDATA 



463 



Following upon the fourth thoracic are about twelve 

 vertebrae all fused into a single mass (Fig. 260, s. scr.\ and 

 giving attachment laterally to the immense pelvic girdle. 

 The whole of this group of vertebrae has, therefore, the 

 function of a sacrum, differing from that of a Reptile in the 

 large number of vertebrae composing it. The first of them 

 bears a pair of free ribs, and 

 is, therefore, the fifth or last 

 thoracic (th. v. j). The next 

 five or six have no free ribs, 

 and may be looked upon as 

 lumbar (Fig. 262, /. i s. j). 

 Next come two sacral vertebrae 

 (c. i) homologous with those of 

 the Lizard. The remaining five 

 vertebrae of the pelvic region 

 are caudal. Thus the mass of 

 vertebrae supporting the pelvic 

 girdle in the Pigeon is a com- 

 pound sacrum, or syn-sacrum, 

 formed by the fusion of the 

 posterior thoracic, all the lum- 

 bar and sacral, and the anterior 

 caudal vertebrae. 



The syn-sacrum is followed 

 by six free caudals and the 



vertebral column ends posteriorly in an upturned, compressed 

 bone, the pygostyle or ploughshare-bone (Fig. 260, pyg. st.), 

 formed by the fusion of four or more of the hindermost 

 caudal vertebrae. 



The sternum (Fig. 260, st.) is one of the most character- 

 istic parts of the Bird's skeleton. It is a broad plate of bone 

 produced ventrally, in the sagittal plane, into a deep keel or 



FIG. 262. Columba livia. Sacrum 

 of a nestling (about fourteen 

 days old), ventral aspect, c 1 . 

 centrum of first sacral vertebra ; 

 c 1 . centrum of fifth caudal ; c. t. 

 first sacral rib ; fl. centrum of 

 first lumbar ; A of third lumbar ; 

 s 1 . of fourth lumbar ; s s . of sixth 

 lumbar; tr.p. transverse process 

 of first lumbar ; tr.p' . of fifth 

 lumbar; tr.p/ 1 . of first sacral. 

 (From Parker's Zoototity.) 



