204 ELEMEJJTART BIOLOGY. [XIII. 



b. The vertebral column. 



It consists of an anterior segmented portion (each 

 segment being a vertebra) and of a posterior 

 unsegmented portion (the urostyle). 



1. Examine carefully and draw various aspects of a de- 

 tached vertebra, say the third. 



a. Its solid flattened ventral part {centrum); with 

 an anterior concave and a posterior convex surface. 



h. The neural arch : an arch of bone springing from 

 the sides of the dorsal aspect of the centrum of 

 the vertebra and not quite so wide as the cen- 

 trum in its antero-posterior diameter. 



«. The transverse process : a bony bar on each 

 side, arising from the arch and passing outwards 

 and a little downwards. 



yS. The articular processes {zygapophyses) ; an an- 

 terior and posterior pair, springing from the 

 sides of the arch ; the anterior, having their 

 smooth articular surfaces directed upwards ; 

 the posterior with similar surfaces directed 

 downwards. 



</. The short spinous process, springing from the 

 dorsal aspect of the arch and directed back- 

 wards. 



c. The neural canal inclosed between the arch and 

 the body. 



2. Examine the remaining vertebrae. 



a. The first vertebra (atlas) : its body produced 

 forwards into a wedge-shaped process which lies 



