XIII.] THE FKOG. 207 



c. The dorsal surface of the spinal column. 



a. The ridge down its middle formed by the row 

 of spinous processes, and continued behind 

 by the dorsal ridge of the urostyle. 



^. The lateral prominences caused by the articular 

 processes : the articulations between the anterior 

 articular processes of one vertebra, and the 

 corresponding posterior processes of its pre- 

 decessor. 



7. The spaces left between the dorsal part of each 

 neural arch and its successor: between the 

 atlas and the second vertebra and between 

 the eighth and ninth vertebrae ; these are almost 

 obliterated by the approach of the respective 

 arches. 



d. The neural canal. 



a. Closed by the centra of the vertebrae beneath, 

 and incompletely by the neural arches on the 

 sides and above : its backward continuation as 

 the canal in the front part of the ridge of the 

 urostyle. 



/3. The communication with it, of the inter- 

 vertebral foramina (4. b. y), and the dorsal 

 intervals between the neural arches (4. c. 7) : 

 also of the openings in the urostyle (3. e). 



c. The skull. The prepared bony skull of the frog 

 is difficult to understand, for two reasons ; firstly, 

 on account of the dried-up condition of the carti- 

 lages of which, in the fresh state, it is in part com- 

 posed; and, secondly, on account of the tendency of 

 many of its constituent bones to become ankylosed 



