PHYLUM CHORDATA 



375 



tr.p 



Following upon the fourth or fifth thoracic are about twelve 

 vertebrae, all fused into a single mass (Fig. 1038, 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 vertebras 

 composing it. The first of them bears a pair of free ribs, and is, 

 therefore, the fifth or sixth (last) thoracic (th.v.5). The next five 

 or six have no free ribs, and may be looked upon as lumbar 

 (Fig. 1040, I. 1 - s. 3 ) : their transverse processes arise high up on 

 the neural arch, and the ligament uniting 

 them is ossified, so that the lumbar region 

 presents dorsally a continuous plate of 

 bone. Next come two sacral vertebras (c. 1 ) 

 homologous with those of the Lizard : be- 

 sides transverse processes springing from *' 

 the neural arch, one or both of them bears 

 a second or ventral outgrowth (c.r.) spring- 

 ing from each side of the centrum and 

 abutting against the ilium just internal to 

 the acetabulum. These distinctive pro- 

 cesses are ossified independently and repre- 

 sent sacral ribs. The remaining five verte- 

 brae of the pelvic region are caudal. 

 Thus the mass of vertebras supporting the 

 pelvic girdle in the Pigeon is a compound 

 sacrum, or syn-sacrum, formed by the fusion 

 of the posterior thoracic, all the lumbar and 

 sacral, and the anterior caudal vertebras. 



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. 1038, pyg.st.}, formed by the fusion of four or more of the 

 hindmost caudal vertebras. 



Thus the composition of the vertebral column of the Pigeon may 

 be expressed in a vertebral formula as follows : 



Syn-sacrum. 



FIG. 1040. Columba livia. 



Sacrum of a nestling (about 

 fourteen days old), ventral 

 aspect, c 1 . centrum of first 

 sacral vertebra ; c". centrum 

 of fifth caudal ; c.r. first 

 sacral rib ; I 1 , centrum of first 

 lumbar ; I-'-, of third lumbar ; 

 s l , of fourth lumbar ; s s , of 

 sixth lumbar ; tr.p. trans- 

 verse process of first lum- 

 bar ; tr.p'. of fifth lumbar ; 

 tr.p". of first sacral. (From 

 Parker's Zootumi/.) 



Cerv. 14. Thor. 4 or 5 + 1. Lumb. 5 or 6. Sacr. 2. Caucl. 5 + 6 + 4 = 43. 



The sternum (Fig. 1038, st.) is one of the most characteristic parts 

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

 ventrally, in the sagittal plane, into a deep keel or carina sterni 

 (car.), formed, in the young Bird, from a separate centre of ossifica- 

 tion. The posterior border of the sternum presents two pairs of 

 notches, covered, in the recent state, by membrane ; its anterior 

 edge bears a pair of deep grooves for the articulation of the 

 coracoids. 



A A 2 



