PHYLUM CHORDATA 



465 



I p 



fe^ 



-^ 



mm- 



and the crocodile, but formed of bone and ankylosed with 

 the rib. 



Following upon the fourth thoracic are about twelve 

 vertebrae all fused into a single mass (Fig. 276, s. scr), and 

 giving attachment laterally to the 

 immense pelvic girdle. The 

 whole of this group of vertebra; 

 has, therefore, the function of a 

 sacrum, differing from that of a 

 reptile in the large number of 

 vertebra? 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. 278, /' — r)- 

 Next come two sacral vertebra? 

 (t~ ] ) homologous with those of 

 the lizard. The remaining five 

 vertebrae of the pelvic region are 

 caudal. Thus the mass of ver- 

 tebrae supporting the pelvic girdle 

 in the pigeon i r ; 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 i: followed by six free caudals and the 

 vertebral column ends posteriorlv in an up turned, compressed 

 bone, the pygostyk or ploughshare-bone (Fig. 276, pyg. si), 

 formed by the fusion of four or more of the hindermost 

 caudal vertebrae. 



The sternum (Fig. 276, si) is one of the most character- 



2H 



Fig 278. - Columba livia. Sac- 

 rum of a nestling .about 

 fourteen days old), ventral 

 aspect, c 1 , centrum of first 

 sacral vertebra; c 1 . centrum of 

 fifth caudal: c. r, first sacral 

 rib; /', centrum of first lumbar; 

 P, of third lumbar; s 1 , of fourth 

 lumbar; J 3 , of sixth lumbar; 

 ir. p, transverse process of 

 first lumbar; tr. p\ of fifth 

 lumbar: tr. p" , of first sacral. 

 (From Parker's Zootomy.) 



