XIII 



PHYLUM CHORDATA 



359 



n.ct 



en, 



Fi<;. 9(50. Columba livia. 



Cervical vertebra. A, anterior ; 

 B, posterior face. a. zyy. an- 

 terior zygapophysis ; en. cen- 

 trum ; 71. a. neural arch ; />. zyg. 

 posterior zygapophysis ; /. rib ; 

 crb. f. vertebrarterial foramen. 



of the neck. They all bear ribs, each consisting of a vertebral (vr.r.) 



and a sternal (st.r.) portion, and articulating with the vertebra 



by a double head. The sternal, like the vertebral rib, is 



formed of true bone, not of calcined 



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 uncinatr, 



(unc.), resembling that of Hatteria and 



the Crocodile, but formed of bone and 



ankylosed with the rib. 



Following upon the fourth thoracic 



are about twelve vertebras all fused into 



a single mass (Fig. 968, 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 tlmt 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. 5). The next 



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



(Fig. 970, 1. 1 s. 3) : their tranverse 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 vertebrae (c.l) 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 independently and represent 

 sacral ribs. The remaining five vertebrae 

 of the pelvic region are caudal. Thus 

 the mass of vertebra 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 lumbar and sacral, and the anterior 

 caudal vertebra. 



The syn-sacrum is followed by six free 

 caudals, and the vertebral column ends 

 posteriorly in an upturned, compressed 



Fio. 970. Columba livia. 



Sacrum of a nestling (about 

 fourteen days old), ventral 

 aspect, c 1 . centrum of first 

 sacral vertebra ; el. centrum of 

 fifth caudal ; c. r. first sacral 

 rib ; 1. centrum of first lumbar ; 

 1^. of third lumbar ; s 1 , of fourth 

 lumbar ; s 3 , of sixth lumbar ; 

 //. p. transverse process of first 

 lumbar ; t r. // of fifth lumbar ; 

 tr. p". of first sacral. (From 

 Parker's Zootomy.) 



