THE VERTEBRAL COLUMN. 3 



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

 body (corpus) (/^) of the vertebra. The centrum is a semi- 

 cyHnder, 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.— FouRiii Thoracic Verte- Fig. 3. — Fourth Thoracic Verte- 



liRA, Cranial End. bra, Side View. 



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

 e, radix or pedicle; /. lamina; g, transverse process; /i, cranial articular facet; i, 

 caudal articular facet; J, caudal articular process; k, 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 

 (r). 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, e), 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, k) for attachment of 

 muscles. 



