PHYLUM CHORDATA 



387 



cartilage as in Reptiles, and articulates with the vertebral rib by a 

 synovial joint. Springing from the posterior edge of the vertebral 

 rib is an uncinate {unc), resembling that of Sphenodon and the 

 Crocodile, but formed of bone and ankylosed with the rib. 



Following upon the fourth or fifth thoracic are about twelve 

 vertebrae all fused into a single mass (Fig. 1027, 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. 1029, 1. Is. 3) : their trans- 

 verse processes arise high up on the neural 

 arch, and the ligament uniting them is 

 ossified, so that the lumbar region pre- 

 sents dorsally a continuous plate of bone. 

 Next come two sacral vertebrae (c.i) 

 homologous with those of the Lizard : 

 besides transverse processes springing 

 from the neural arch, one or both of 

 them bears a second or ventral outgrowth 

 (c.r.) springing from each side of the 

 centrum and abutting against the ilium 

 just internal to the acetabulum. These 

 distinctive processes are ossified inde- 

 pendently and represent sacral ribs. The 

 remaining five vertebrae of the pelvic 

 region are caudal. Thus the mass of 

 vertebrae supporting the pelvic girdle 

 in the Pigeon is a compound sacrum, or 

 syn-sacrmn, 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. 1027. pyg.st.), formed by the 

 fusion of four or more of the hindmost caudal vetebrse. 



Thus the composition of the vertebral column of the Pigeon may 

 be expressed in a vertebral formula as follows : — 



Fig. 1029.— Columba livia. 



Sacrum of a nestling (about 

 fourteen days old), ventral 

 .aspect, c^. centrum of first 

 sacral vertebra ; cl.rcentrum of 

 fifth caudal ; c. r. first sacral 

 rib; ^1. centrum of first lumbar ; 

 Z3, of third lumbar ; si, of fourth 

 ^ lumbar ; a3, of sixth lumbar ; 

 tr. ■p. transverse process of first 

 lumbar ; tr. p'. of fifth lumbar ; 

 tr.p.". of first sacral. (From 

 Parker's Zootomy.) 



Syn-sacram. 



Cerv. 14. Thor. -4 or 5 + 1. Lumb. 5 or 6. Saor. 2. Caud, 5 



Pyg- 



6 + 4 =43. 



The sternum (Fig. 1027, s^.) is one of the most characteristic 

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



