XIII THE SKELETON 



237 



in which ossifications of membranes, or dermal bones, be- 

 come appUed to the cartilaginous cranium, while the latter 

 remains in great part unossified. In the birds and mam- 

 mals ossification of the original cartilaginous basis of the 

 cranium has become almost complete, and the bones that 

 are developed from membrane enter into more intimate rela- 

 tions with the cartilage bones than they do in the frog, the 

 whole forming a structure which is very firm and compact. 



The Vertebral Column. — The vertebral column of the 

 frog consists of ten bones of which the first nine are verte- 

 brae proper, the tenth, or urostyle, being a long rodlike 

 bone extending from the ninth vertebra to the apex of the 

 pelvic girdle. A typical vertebra of which we may take the 

 third as an example consists of the following parts : — 



(i) The centrum, a basal portion, which is oval in cross 

 section and concave in front for the reception of the cen- 

 trum of the preceding vertebra and convex posteriorly. 



(2) The neural arch, which incloses the neural canal, in 

 which is lodged the spinal cord. The neural arch is pro- 

 duced in the mid-dorsal line into a projection called the 

 neural spine, and at the sides it bears a pair of elongated 

 transverse processes which extend almost at right angles to 

 the body ; the tips of these processes are furnished with car- 

 tilaginous epiphyses. Both the posterior and anterior mar- 

 gins of the neural arch bear a pair of short articulating 

 processes or zygapophyses, by means of which the successive 

 vertebrae are joined together; the articular faces of the 

 anterior articulating processes look upward and inward, and 

 are covered by the posterior zygapoj)hyses of the preceding 

 vertebra, the articulating faces of which look downward and 

 outward. The opposed faces of the adjoining zygapophyses 

 are smooth and allow a limited gliding movement when the 

 spinal column is bent from side to side. 



