VERTEBRATES. 



13 



is observable in the greater distinctness of the regions into which the column is divisi- 

 ble in the higher forms. In a fish one distinguisiies between trunk-vcrtcbne and tail- 

 vertebra?, the latter 

 being in that ]iart of 

 the body behind the 

 ccelom, while in most 

 higher forms (Fig. 

 13) five regions are 

 distinguishable. 

 Thus cervical verte- 

 brie are distinguish- 

 ed from the succeed- 

 ing trunk vertebrie 

 by the fact that their 

 ribs (when present) 

 do not unite ventral- 



ly in a sternum. Again, certain sacral vertebrse are specialized by their ribs affording 

 su])port to the pelvic girdle, which, in its turn, supports the hind limbs. Behind the 

 sacral region is the caudal region ; in front of it, the trunk, or dorso-lumbar region, 

 composed of the dorsal and lumbar VL-rtebr;e, the latter differing from the former in 

 having no movable ribs. 



Throughout the vertebrates we meet with parts of the vertebral coh;mn, modified 

 in coiniection with requirements of the most various character. The union with the 

 head modifies the first (atlas) or first two vertebra3 (atlas and axis). In some Teleosts 

 certain anterior vertebne are altered in form to establish communication between the 

 air-bladder ;nid the ivir ; and in all, the end of the caudal region is modified in connec- 

 tion with th^' caudal tin. Again, fusion of certain tracts is normal in the sacrum, and 

 may be exceptionally present in other regions, as the tip of the tail in birds, the dorsal 

 region in turtles, etc. 



Fig. 13.— Skeleton of bo.w : 



Toe Ribs and Steexum. 



Tiie mode in which the ril« of the gar-pike are developed, continuously with the 

 ha?mal arches, has been referred to above. Whether the ribs of the higher forms, 

 which frequently unite in the v^entral mid<lle 

 line to form a sternum, are homologous struc- 

 tures, is not yet completely determined. 

 That the second point of contact obtained 

 with the neural arch of the \'ertebra is a 

 new acquisition of the higher forms has been 

 hinted at above. 'In defining the regions of 

 the vertebral column of the higher forms, it 

 has been indicated that in these the I'ibs are 

 very different in different regions. Cervical fk;. h.— section of the thoracic region of the skeie- 



'i 1 111.., II ton of a crocodile ; r, centrnin of vertebra; rr, car- 



nbS may be movable, but they are generally Ulaginous portion of vertebral rib; n, neural spine: 



I. 4. ., 1 J.' 1 -xi ..u' x'l ii '.. «, uncinate process on r, vertebral rib ; s. sternum ; 



so Stiort and so fused with the vertebra, that sr, sternal rib; t, transverse process; z, articular 



they merely form a ' jjerfoi-ated ' transverse p™'^^*^- 



process. Dorsal ribs, on the other hand, as alre;idy mentioned, unite to form the 



sternum; this they do by their ventral ends fusing together into two cartilaginous 



