VEETEBEAL COLUMN 45- 



It is traversed from its cephalic end to the cephalic part of the tail 

 by the vertebral or neural canal, wherein is lodged the spinal cord 



(Fig. 14). 



The column, therefore, is a bony tube with projections for muscu- 

 lar attachment. It is bilaterally symmetrical, but for greater rigidity 

 the ventral wall is thickened into a rod, while the lateral and dorsal 

 walls remain thin. 



The vertebral column is not truly a rod, but is broken by .joints 

 into a series of small segments arranged end to end. These segments 



FIG. 14. 



Skull, x-^^^y 



\~n-tebral Column. 



THE AXIAL SKELETON. LONGITUDINAL SECTION. 

 Shewing relations of Neural, Oral, and Thoracic Cavities. 



are bound closely together, and are more or less movable on one 

 another at the joints. Their close union permits of considerable range 

 of motion to the column as a whole, but guards against disturbance 

 of the spinal cord from extreme motion at any one point. 



The segments into which the column is divided are called the 

 vertebrae, and, since the column is a tube, each vertebra is a ring or 

 section of a tube. These rings are of varying widths and lengths. 

 They are always bilaterally symmetrical ; the ventral part, however, 

 is thickened into a cylindrical mass articulating in front and behind 

 with the adjoining vertebrae. The dorsal part of the ring, in the form 

 of a bony arch, surrounds the neural canal. 



In number, the vertebrse vary from forty-eight to fifty-three, ac- 

 cording to the length of the tail. They are divided into five groups 

 (Fig. 15), as follows : 



The Cervical l Vertebrae, the cephalic seven forming the neck ; 



The Thoracic 2 Vertebrae, the following thirteen, which unite with 

 the ribs to complete the thorax ; 



1 From cervix, the neck. 2 From thorax (Gr.), a breastplate ; the breast. 



