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 distinguishes between trunk-vertebrae and tail- 



vertebrse, the latter j\ ,. ^ a 



being in that part of 

 the body behind the 

 coelom, while in most 

 higher forms (Fig. 

 13) five regions are 

 distinguishable. 

 Thus cei'vical verte- 

 brae are distinguish- 

 ed from the succeed- 

 ing trunk vertebrae 

 by the fact that their 

 ribs (when present) 

 do not unite veutral- 



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

 support 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 vertebrae, the latter differing from the former in 

 having no movable ribs. 



Throughout the vertebrates Ave meet with parts of the vertebral column, modified 

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

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

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

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

 tion with the caudal fin. Again, fusion of certain tracts is normal in the sacrum, and 

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

 region in turtles, etc. 



Fig. 13.— Skeleton of boar ; c, cervical ; </, dorsal ; I, lumbar ; s, sacral, and t, caudal 



vertebrae. 



The Ribs and Stbenum. 



The mode in which the ribs of the gar-pike are developed, continuously with the 

 haemal arches, has been referred to above. Whether the ribs of the higher forms, 

 which frequently unite in the ventral middle 

 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 vertebra 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 ribs are 

 very different in different regions. Cervical 

 ribs may be movable, but they are generally 

 so short and so fused with the vertebra, that 

 they merely form a 'perforated' transverse 

 process. Dorsal ribs, on the other hand, as already mentioned, unite to form the 

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



Fig. 14.— Section of the thoracic region of the skele- 

 ton of a crocodile ; c, centrum of vertebra ; cr^ car- 

 tilaginous portion of vertebral rib ; n, neural spine ; 

 p, uncinate process on r, vertebral rib ; s, sternum ; 

 sr, sternal rib ; t, transverse process ; z, articular 

 process. 



