[Illustration: FIG. 2.--FOURTH THORACIC VERTEBRA, CRANIAL END.
FIG. 3.--FOURTH THORACIC VERTEBRA, SIDE VIEW.
_a_, vertebral foramen; _b_, centrum; _c_, caudal, and _d_, cranial, costal demifacets; _e_, radix or pedicle; _f_, lamina; _g_, transverse process; _h_, cranial articular facet; _i_, caudal articular facet; _j_, caudal articular process; _k_, spinous process.]
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 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 vertebræ are placed together in the natural position the cranial costal demifacets of one together with the caudal demifacets 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 =vertebral arch= which is continued dorsally into the long, bluntly pointed =spinous process= (Figs. 2 and 3, _k_) for attachment of muscles.
The vertebral arch (each half of which is sometimes called a =neurapophysis=) rises on each side from the cranial two-thirds of the dorsolateral angle of the centrum, as a thickened portion, the radix or pedicle (Figs. 2 and 3, _e_), which forms the ventral half of the lateral boundary of the vertebral canal. From the dorsal end of each radix a flat plate of bone, the =lamina= (_f_), extends caudomediad to join its fellow of the opposite side and form the vertebral arch. Owing to the fact that the radix rises from only the cranial two-thirds of the centrum there is left in the caudal border of the vertebral arch a notch bounded by the radix, the lamina, and the centrum. There is also a slight excavation of the cranial border of the radix. When the vertebræ are articulated in the natural position, these notches form the =intervertebral foramina= (Fig. 4, _d_), for the exit of the spinal nerves.
At the junction of radix and lamina the arch is produced craniolaterad into a short process, the =transverse process= (_g_), knobbed at the end. On the ventral face of its free end the transverse process bears a smooth facet, the transverse costal facet or tubercular facet (Fig. 4, _c_), for articulation with the tubercle of a rib.