VERTEBRAL COLUMN. 285 



I. THE AXIAL SKELETON. 



A. The Vertebral Column or ' backbone.' 



This consists of a series of bony rings or vertebrae, placed 

 end to end so as to form a tube, which stiffens the back, and 

 lodges and protects the spinal cord. The successive vertebrae 

 are movably articulated with one another, and their centra 

 are separated by intervertebral discs of fibro-cartilage, except 

 in the sacrum, where a certain number are fused together to 

 support the pelvis. 



The vertebrae fall readily into five groups, named as follows 

 from before backwards : — 



(1) The cervical or neck vertebras. 



(2) The thoracic or chest vertebrse, which bear movably 

 articulated ribs. 



(8) The lumbar vertebrae, which are the largest of the 

 series, and bear no ribs. 



(4) The sacral vertebrae, which are fused together and 

 support the pelvis. 



(5) The caudal or tail vertebrae. 



1. General characters of a vertebra. 



Examine, as a type, the second lumbar vertebra. 



a. The vertebra is tubular, the cavity or neural canal 



being traversed during life by the spinal cord. 



b. The centrum or body is the thickened ventral wall 



of the tube, forming the floor of the neural canal. 

 It corresponds in position to the notochord of the 

 embryo, which it replaces (c/. p. 214). 



c. The epiphyses are two thin plates of bone applied 



to the ends of the centrum, with which they fuse 

 after the animal is full grown. 



d. The neural arch forms the sides and roof of the 



neural canal, and presents at each end, close to 

 the centrum, a pair of intervertebral notches for 



