THE INTERNAL SKELETON. 169 
Tur Bacx-Bone, Rips, AND BREAST-BONE, 
From 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 
vertebra, for which reason the term vertebral column is also 
applied to the whole spine. 
The “ vertebra ” being thus serially conjoined, the juxtaposed 
Fig. 148. 

DIAGRAM OF A VERTEBRA, seen in front, 
The shaded central part in the middle is the centrum of the vertebra. 
D. Diapophysis or superior transverse process. H. Hiemal arch. HC, 
Hemal canal enclosed by hemal arch. Ay. Diverging ends of hypa- 
pophyses. | N. Neural canal enclosed by neural arch. NA, Neural 
arch. NS. Neural spine. P. Parapophysis or inferior transverse 
process. P/, Pleurapopbysis, 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 itis an arch springing by piers from either side of 
the centrum, 
