THE VERTEBRAL COLUMN 43 



b. Special vertebrae. 



i. The atlas or first vertebra 'articulates in front 

 with the posterior end of the skull: it 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 de- 

 stroyed in the operation of pithing a frog. 



ii. The sacrum, or ninth vertebra, has very stout 

 backwardly directed transverse processes, which 

 support at their outer ends the pelvic arch. 



c. The urostyle is the unsegmented posterior portion of 



the vertebral column. 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 intervertebral 

 foramina. 



II. The Skull. 



The skull consists of, (1) an axial portion, the cranium, 

 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 (4) the hyoid apparatus. 



In the skull the original cartilage, or chondrocranium, 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. 



1. The Cranium is originally an unsegmented cartilaginous 

 tube, whose cavity forms the anterior part of the 

 neural canal, and lodges the brain. The roof of the 



