ii.j THE SPINAL SKELETON. 39 



The division of the vertebras into true and false is seen to 

 be a very arbitrary one when we extend our view, as all are 

 "true" in Serpents, and those which are "false" in man are 

 true even in some members of his own class, e.g. the 

 Cetaceans. On the other hand, many vertebrae which are 

 reckoned to be "true" in him are "false" in other animals. 

 Thus, in Birds the process of anchylosis invades the lumbar 

 and dorsal vertebrae. In Tortoises all the trunk vertebrae are 

 fused, and therefore "false," while in the Glyptodon none 

 of the vertebrae except the coccygeal can be said to be 

 " true " ones. 



Again, vertebrae may be more thoroughly " false " than even 

 in man, as in the sacrum of the Rhea (or American 

 Ostrich), where between the hip-bones they abort, and are 

 represented only by a long narrow strip of bone. Distinct 

 vertebrae are developed both pre-axially and post-axially to 

 this strip. "Degradation" is a constant character of the 

 last vertebrae in all classes of Vertebrates. 



22. As to VERTEBRAE in GENERAL, the neural arch is the 

 constant character of a vertebra persisting even where (e.g. 

 in Lepidosireri) the bodies are not formed. 



Two neural arches may correspond to one centrum, as 

 in some Elasmobranchs ; and merely cartilaginous neural 

 arches may exist, as in the Sturgeon. The arch may present 

 processes which are not developed in man. The two laminae 

 may (as on the Axolotl) fail to meet together on the dorsal 

 aspect. 



The centrum, or body, may have its opposite surfaces 

 strongly convex or concave, contiguous vertebrae uniting by a 

 ball and socket joint, instead of being flat or nearly so, as 

 in man. 



The ball may be post-axial in each vertebral body, a struc- 

 ture termed proccelous? and found e.g. in existing crocodiles ; 

 or the ball may be pre-axial, which condition is called opis- 

 thoccelous? and is more rare, but is found e.g. in the land 

 Salamander, and even in man's own class, as in the cervical 

 vertebrae of the Ruminants. 



The vertebrae may have both surfaces hollow, a structure 

 called amphicoelous ; 3 or bi-concave, a condition found in 

 most Fishes, and even some Reptiles, as the Geckoes. The 

 osseous bodies may be reduced to mere rings encircling the 



1 From vrpo, before, and KoIXog, hollow. 



2 From oTTia-Oe, behind, and KoiAo^. 



3 From a'jutf><, both, and KIH\O. 



