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. On the dorsal face of each lamina at its cranial border is a smooth oval area, the =cranial articular facet= (superior articular facet of human anatomy) (Figs. 2 and 3, _h_). Its long axis is oblique and it looks dorsolaterad. The slight projections of the cranial edge of the laminæ on which the facets are situated are the inconspicuous =cranial articular processes= (prezygapophyses). On the ventral surface of each lamina at the caudal border, near the middle line is a similar area, the =caudal articular= =facet= (inferior articular facet of human anatomy) (_i_); these occupy the ventral surfaces of two projections which form the =caudal= (inferior) =articular processes= (postzygapophyses) (_j_). These are separated by a median notch. When the vertebræ are in their natural position the caudal articular facets lie dorsad of the cranial facets and fit against them. They thus strengthen the joint between contiguous vertebræ, while permitting slight rotary motion. [Illustration: FIG. 4.--THORACIC VERTEBRÆ, SIDE VIEW. _a_, spinous processes; _b_, cranial articular processes; _c_, transverse costal facets; _d_, intervertebral foramina; _e_, costal facets; _f_, accessory processes; _g_, mammillary processes; _h_, caudal articular processes.]