52 THE FROG 



A. The Axial Skeleton. i. The vertebral column con- 

 sists of nine vertebrae and a posterior unsegmented portion, 

 the coccyx or so-called urostyle. 



(a) Select a typical vertebra, for example the fourth, 

 for careful study. Observe that it forms a bony ring, en- 

 closing a central neural canal which during life is occupied 

 by the spinal cord. The floor of the canal is formed by the 

 centrum of the vertebra, which in the complete vertebral 

 column is joined to the centra of adjacent vertebrae by 

 means of pads of fibre-cartilage. The neural arch com- 

 prises the lateral and dorsal portions of the ring and forms 

 the sides and roof of the neural canal. Projecting dorsally 

 and posteriorly from the top of the neural arch is the 

 spinous process or neural spine. The transverse proc- 

 esses are a pair of large projections extending horizontally 

 outwards from the sides of the neural arch. The two pairs 

 of articular processes or zygapophyses, situated respec- 

 tively on the anterior and posterior borders of the neural 

 arches, articulate with corresponding processes of adjacent 

 vertebrae; the anterior articular processes or prezyga- 

 pophyses face upward and slightly inward, while the pos- 

 terior articular processes or postzygapophyses face 

 downward and slightly outward. Draw a posterior view of 

 a vertebra (X 4). 



When all its parts are in place the vertebral column forms 

 a nearly complete tube for the protection of the spinal cord, 

 but at the sides of this tube, between the successive vertebrae, 

 are openings, the intervertebral foramina, through which 

 nerves pass out from the spinal cord. 



(&) Special vertebrae. The first vertebra or atlas ar- 

 ticulates with the skull; how? This vertebra has no trans- 



