THE VERTEBRAL COLUMN. 



third of this ring is much thickened and forms the centrum or 

 body (corpus) (b} 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 



FIG. 2. FOURTH THORACIC VERTE- FIG. 3. FOURTH THORACIC VERTE- 



BRA, CRANIAL END. BRA, SIDE VIEW. 



a. vertebral foramen; b, centrum; c, caudal, and d, cranial, costal demifacets;. 

 e, radix or pedicle; f. lamina; g, transverse process; A, cranial articular facet; t r 

 caudal articular facet ; /, caudal articular process ; A, 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 

 cpiphysis and bounded dorsolaterally by a sharp ridge of bone 

 (f). It is a costal demifacet. In corresponding positions at 

 the cranial end of the centrum are two demifacets not limited 

 by bony ridges (d}. When the centra of two contiguous 

 thoracic vertebrae 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, c), one ort 

 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, k} for attachment of 

 muscles. 



