=The Atlas= (Fig. 10, 1; Fig. 12).--The first cervical vertebra or atlas has somewhat the form of a seal ring. The centrum is absent; it has united with the second vertebra to form the odontoid process or dens. Its place is taken in the atlas by a narrow flat arch of bone, narrower at the ends than in the middle, the =ventral arch= (Fig. 12, _a_) of the atlas. This connects the lateral, thicker portions of the ring ventrally and bears on its caudal margin a blunt tubercle (=tuberculum anterius=, Fig. 12, _b_). Laterally the ring is thickened, forming thus the =lateral masses= (_c_) which are continued into the broad thin transverse processes (Fig. 10, _g_; Fig. 12, _d_). Each lateral mass bears at its cranial end on its medial surface a concave, pear-shaped facet, cranial (or superior) articular facet, (Fig. 12, _e_) for articulation with the condyles of the skull. These facets look craniomediad. Dorsad of each is a foramen, the atlantal foramen (Fig. 10, _i_; Fig. 12, _g_), which pierces the dorsal arch at its junction with the lateral mass. Caudal to the facet, on the medial face of each lateral mass, within the vertebral canal, is a tubercle. To the two tubercles are attached the transverse ligament (Fig. 14, _b_) which holds in place the odontoid process (dens) of the axis. That part of the lateral mass which bears the articular facet projects craniad of the dorsal arch and is separated by a deep triangular notch from the transverse process. Along the bottom of this notch runs a groove (Fig. 12, _f_), convex craniad, which connects the cranial end of the foramen transversarium and the atlantal foramen. The vertebral artery passes along it. The foramen transversarium is circular. It is bounded laterally by the lateral masses, and dorsally by the dorsal arch. The dorsal arch (Fig. 10, _h_) is two to three times as broad as the ventral, has a thick convex cranial border with a median notch, and a thin concave caudal border. [Illustration: FIG. 13.--AXIS OR EPISTROPHEUS, SIDE VIEW. _a_, odontoid process or dens; _b_, cranial articular facets; _c_, spinous process; _d_, caudal articular facet; _e_, transverse process; _f_, foramen transversarium.] The caudal articular facets (Fig. 12, _h_) are borne by the caudal ends of the lateral masses. They are slightly concave, triangular, and look caudomediad, so that their dorsal borders form with the caudal border of the dorsal arch nearly a semicircle. The transverse processes are flat and directed laterad. The attached margin of each is about two-thirds the length of the thinner free margin. The somewhat thicker caudal end of the transverse process projects further caudad than any other part of the vertebra and is separated by a slight notch from the caudal articular facet. From the bottom of this notch the foramen transversarium extends craniad and opens at the middle of the ventral face of the transverse process.