48 



VERTEBRATE SKELETON 



additional strength to the thoracic basket. No living birds have free 

 lumbar vertebra?, these being included in the synsacrum (fig. 53). 

 The two true sacrals (three in Strut/no and Apteryx), lying just behind 

 the fossse for the kidneys, are united to the greatly elongate ilium by 

 sacral ribs. The number of synsacrals is three in ArchcEopteryx, 

 the lowest in living birds is nine and there may be twenty (casso- 

 wary) . The bipedal life, the obliquity of the body axis and the neces- 

 sity of a firm attachment of vertebral 

 column and pelvis are the causes of the 

 synsacrum. 



In all modern birds the tail is very 

 short, but is long and lizardlike in Arch- 

 CEopteryx, with about twenty elongate 

 centra, movably articulated with each 

 other. Hesperornis has twelve centra, the 

 last few being fused to an imperfect 

 pygostyle. In modern birds there are 

 usually five or six free caudals followed 

 by four to six (free in the embryo) fused 

 to a single mass, the pygostyle (urostyle, 

 ploughshare bone). 



MAMMALIA. — There has been no 

 recent work on the ontogeny of the mam- 

 malian vertebrae; it is known that the 

 arches belong to the caudal half sclero- 

 tomes, but not to what extent the cranial 

 halves contribute. It is probable that the 

 latter form a considerable part in the tail, 

 although the neural arch springs from most of the length of 

 the centrum. In development chondrification extends to the 

 intervertebral tissue, the column for a time being a cartilage contin- 

 uum, only separated into vertebrae at the time of ossification which 

 begins on the inside of the cartilage (endochondrostosis) without 

 the external osseous plates which are so common in many lower Ver- 

 tebrates. The arches ossify first, and there are separate centres 

 for epiphyses, transverse processes, zygapophyses and neural spine, 

 the phylogenetic significance of which is unknown. 



In all mammals (except Cetacea and Sirenia which lack a sacrum) 

 all five vertebral regions occur. The usual number of vertebra? is 



Fig. 53 . — Synsac rum and 

 pelvis of hawk, il, ilium; is, 

 ischium; p, pubis; pp, pectineal 

 process; 5, sacral ribs. 



