=Thoracic Vertebræ.= =Vertebræ thoracales= (Fig. 4).--The =thoracic vertebræ= are most typical, and the fourth one of these may therefore be first described (Figs. 2 and 3). It forms an oval ring which has numerous processes and surrounds an opening which is the vertebral foramen (_a_). The ventral one-third of this ring is much thickened and forms the =centrum= or body (=corpus=) (_b_) of the vertebra. The centrum is a semicylinder, 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 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=. [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.