52 THE SKELETON OF THE FROG. 



A. The Axial Skeleton. 



I. The Vertebral Column or "back bone." 



This is a bony tube which surrounds and protects the spinal 

 cord ; it consists of an anterior part which is divided trans- 

 versely into nine rings or vertebrae, and a posterior unsegmented 

 portion of about equal length — the urostyle. At the sides of 

 the tube, between the successive vertebrse, are the intervertebral 

 foramina through which the nerves pass out from the spinal 

 cord to the various parts of the body. 



a. Structure of a vertebra. Examine one of the vertehrce, 

 say the third, more closely : draw it, showing the follow- 

 ing points : 



i. The vertebra is a bony ring ; the s]3inal cord lying 

 during life in the central neural canal. 



ii. The centrum or body is the thickened ventral por- 

 tion of the ring : it articulates with the centra of 

 the vertebrae in front of and behind it ; and forms 

 the floor of the neural canal. 



iii. The neural arch consists of the lateral and dorsal 

 portions of the ring ; and forms the sides and 

 roof of the neural canal. 



iv. The spinous process or neural spine is a small blunt 

 median process, projecting upwards and back- 

 wards from the top of the neural arch. 



v. The transverse processes are a pair of large processes 

 projecting horizontally outwards from the sides 

 of the neural arch. 



vi. The articular processes or zygapophyses, on the 

 anterior and posterior borders of the neural arch, 

 articulate with corresponding processes on the 

 vertebrae in front and behind, and so serve to 

 link the vertebrae together. 



a. The anterior articular processes, or praezyg- 

 apophyses, face upwards and slightly in- 

 wards. 



f3. The posterior articular processes, or post- 

 zygapophyses, face downwards and slightly 

 outwards. 



