48 THE SKELETON OF THE FROG. 
b. Peculiar vertebra. 
a 
i. The atlas or first vertebra: articulates in front 
with the posterior end of the skull: has no 
transverse processes. Note the large gap on the 
dorsal surface between the skull and the neural 
arch of the atlas: through this gap, which is 
closed by the strong occipito-atlantal membrane, 
the central nervous system is divided and des- 
troyed in the operation of pithing a frog. 
ii, The sacrum: the ninth vertebra; has very stout 
backwardly directed transverse processes, which 
support at their outer ends the pelvic arch. 
c. The urostyle. The unsegmented posterior portion of 
the vertebral column ; probably equivalent to two or 
more vertebree fused together. It articulates in front 
with the body of the sacral vertebra by two surfaces. 
Along its dorsal surface runs a prominent vertical 
ridge, highest in front and gradually diminishing 
posteriorly: the neural canal is continued down the 
anterior part of this ridge. At the sides of the 
urostyle, and about the length of a vertebra from its 
anterior end, are a pair of small holes through which 
nerves pass out, and which therefore correspond to 
inter-vertebral foramina. 
II. The skull. Consists of (1) an axial portion, the 
cranium proper, enclosing the brain and forming an anterior 
continuation of the vertebral column : (2) the olfactory capsules 
and the auditory capsules, which are fused with the anterior 
and posterior ends of the cranium respectively: (3) the bony 
framework of the jaws, and the hyoid apparatus. 
In the skull the original cartilage is not so largely replaced 
by bone as in the vertebral column, large tracts of unossified 
cartilage persisting in the adult. Besides the cartilage bones 
the skull is further strengthened by the addition of numerous 
membrane bones. 
