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MANUAL OF ELEMENTARY ZOOLOGY 



limb, contain air spaces. A tendency to the fusion of bones 

 is seen in various regions, and the proportion of cartilage 

 is very small. The backbone is divided into five regions : 

 (i) The neck contains thirteen to fifteen cervical vertebrae, 

 the commonest number being fourteen. The ends of the 

 centra of these are of a peculiar shape known as heterocxlous. 

 In front they are saddle-shaped, concave from side to side, 

 and convex from above downwards : behind they have these 

 curvatures reversed. The third to the eleventh or twelfth 

 cervical vertebras bear short ribs fused to the centra and 

 transverse processes. The ribs of the last two are free, but 

 do not reach the breastbone. (2) Behind these come five 

 thoracic vertebras, whose ribs reach the breastbone. Of 



Fig. 294. — Cervical vertebra of a pigeon. 



A, From in front ; B, from behind. 



az., Prezygapophysis ; c.r., cervical rib; cm., centrum; n.a., neural arch;/.z., 

 postzygapophysis ; ver.c, foramen of transverse process. 



these the first three are fused together, the fourth is free, 

 and the fifth is fused with those behind it. (3) The next 

 half-dozen vertebrae are known as lumbars and are fused in 

 front with the last thoracic and behind with (4) the two 

 sacral and (5) the first five caudals. Thus there is a long 

 group of fused vertebrae, known as the sacrum, to which 

 the pelvic girdle is attached. Then follow six free caudals 

 and the, ploughshare bone or pygosty/e, which consists of four 

 fused vertebras and supports the tail. Each rib has a head or 

 capitulum which articulates with the centrum of its vertebra 

 and a tubercle which articulates with the transverse pro- 

 cess. Those which join the sternum are bent forwards at 

 an angle to do so, the part above the angle being known 

 as the vertebral rib, that below as the sternal rib. Both 



