THE INTEENAL SKELETOlTk 1 69 



The Back-bone, Eibs, and Bebast-bone. 



Prom the hinder side of the head to the end of the tail there 

 runs that bony structure already spoken of as the " back-bone " 

 or, as it is also often called, the spinal column. It is made 

 up of a chain of bones placed one behind the other, each being 

 a more or less cylindrical ring, much thicker on one side — the 

 ventral side — than elsewhere. Each of these bones is called a 

 verteWa, for which reason the term vertebral column is also 

 applied to the whole spine. 



The " vertebrse " being thus serially conjoined, the juxtaposed 



j5^ 



BiAQBAM or A Vertebra, seen in front. 



The shaded central part in the middle is the centrum of the rertebra. 



D. Diapophysis or superior transverse process. H. Haemal arch. HC, 

 Hsemal canal enclosed by h^mal areh. Hy. Diverging ends of hypa- 

 pophyses. N. Neural canal enclosed by neural arch. NA. Neural 

 arch. N8. Neural spine. P. Parapophysis or inferior transverse 

 process. Fl. Pleurapophysis, represented as present on one side only. 

 Z. Zygapophysis. 



rings form a long canal called the vertebral or neural canal, 

 because it contains and protects the spinal cord or spinal marrow, 

 the most important part, with the brain, of the nervous or 

 " neural " system. The thickened or ventral part of each ver- 

 tebra is named the centrum, and the rest of the ring the neural 

 arch, for it is an arch springing by piers from either side of 

 the centrum. 



