A VERTEBRA. 



31 



the vertebra ; the series of vertebral bodies forms the bony 

 partition (e, e, Fig. 1) already mentioned as existing in the 

 trunk between the neural and haemal cavities. (2) An 

 arch attached to the dorsal side of the centrum ; it is the 

 neural arch, A, and with the centrum incloses the neural 

 ring (Fv). The vertebrae being piled one above the other 

 the successive neural rings form the neural tube, in the 



Pas 



FIG. 11. 



FIG. 12. 



FIG. 11. A dorsal vertebra seen from behind, i.e., the end turned from the 

 head. 



FIG. 12. Two dorsal vertebrae viewed from the left side, and in their natural 

 relative positions. 6', the body ; A, neural arch ; Fv, the neural ring ; Ps, epi- 

 nous process ; Pas. anterior articular process : Pai, posterior articular process : 

 Pt, transverse process ; Ft, facet for articulation with the tubercle of a rib ; 

 Fes, Fci, articular surfaces on the centrum for articulation with a rib. 



cavity of which the spinal cord lies. (3) Projecting from 

 the body and arch are several processes ; one reaching out 

 from the dorsal side of the arch is the spinous process ; 

 the row of spinous processes which may be felt through 

 the skin along the middle of the back has given the name 

 of spinal column to the whole backbone. 



What constitutes the bard partition between the dorsal and ventral 

 cavities of the trunk? How is the neural tube formed? Why is 

 the spinal column so named ? 



