ii.] THE SPIRAL SKELETON. 53 



The total absence of articular processes is a character 

 common to man's class, but in Birds and in some Reptiles 

 (e.g. the Iguana) postzygapophyses are present. 



The spinous tubercle may be enlarged into a distinct 

 pointed process, as in some Baboons. The neural spine may 

 be detached from the neural arcs, as in the Crocodile and 

 Tunny. It may, as in the fish Ephippus, be separated from its 

 centrum by the intrusion of the skull wall ; the inferior tubercle 

 may be drawn out into a long process, as in the Hare, or be 

 doubled, as in the Duck-billed Platypus. Occasionally, as in 



TIG. 67. LATEKAI., DORSAL, AND VENTRAL VIEW OF FIRST VERTEBRA OF 

 A inpkiuina. 



Amphiuma, the body of the first vertebra may send out 

 a process towards the head, reminding us of the odontoid 

 process of the axis. 



The slenderness of the neural arch and transverse processes 

 is exceptional in man. The arch may be open medianly 

 above, as in the Frog. The transverse process is expanded 

 into a large plate in almost all beasts below Primates, but it 

 may be short and imperforate, as in the common Fin Whale. 



The canal for the vertebral artery is generally more com- 

 plex (through increase of ossification) in beasts than in 

 man, where indeed the osseous canal is simple. Transverse 

 processes are also generally absent or rudimentary below 

 Mammals, though the Crocodiles have them very long and 

 rib-like. 



The free condition of the atlas is not universal, as it is 

 anchylosed to the axis in the Dolphin, and with all the other 

 cervicals in the Right Whale. It may be fused in one solid 

 mass with the skull, as in the Sturgeon, or with a certain 

 number of succeeding vertebras, as in the Rays. It may be 

 firmly united by suture with both, as in the Ganoid fish Bagrtis 

 (see Fig. 61). The anterior base of the atlas may be un- 

 ossified, as in the Wombat, or may remain a distinct bone, 

 as in the Thylacine. The real nature of this part will appear 

 shortly. 



