THE VERTEBRAL COLUMN. 3 
third of this ring is much thickened and forms the centrum or 
body (corpus) (4) of the vertebra. The centrum is a semi- 
cylinder, the plane face of which bounds the vertebral canal, 
while the curved surface is concave longitudinally and is 
directed ventrad. The dorsal plane surface of the centrum is 
marked by a median longitudinal ridge on either side of which 
Fic. 2,—FourTH THORACIC VERTE- Fic. 3.—FourRTH THORACIC VERTE- 
BRA, CRANIAL END. BRA, SIDE VIEW. 
a, vertebral foramen; 4, centrum; ¢, caudal, and d, cranial, costal demifacets; 
e, radix or pedicle; f lamina; g, transverse process; 4, cranial articular facet; 7, 
caudal articular facet; 7, caudal articular process; 4, spinous process. 
is an opening (nutrient foramen) for a blood-vessel. The 
ends are nearly plane, the caudal being slightly concave; they 
are harder and smoother than the other surfaces. They may 
be easily separated in a young specimen as thin plates of bone 
known as epiphyses. 
At the caudal end of the centrum, at its dorsolateral angle, 
is a smooth area on each side continuous with the surface of the 
epiphysis and bounded dorsolaterally by a sharp ridge of bone 
(c). It isa costal demifacet. In corresponding positions at 
the cranial end of the centrum are two demifacets not limited 
by bony ridges (@). When the centra of two contiguous 
thoracic vertebre are placed together in the natural position the 
cranial costal demifacets of one together with the caudal demi- 
facets of the other form two costal facets (Fig. 4, ¢), one on 
each side, and each receives the head of a rib. 
The dorsal two-thirds of the vertebral ring forms the verte- 
bral arch which is continued dorsally into the long, bluntly 
pointed spinous process (Figs. 2 and 3, #) for attachment of 
muscles. 
