The lateral surface may be divided into two parts. Craniad is a large rough triangular area with equal sides and with one of its angles directed ventrocraniad. It is the lateral face of the pseudo-transverse process of the first sacral vertebra ([Fig. 6], c). A smooth curved surface (the auricular facet) along its ventral edge articulates with the ilium, while the dorsal portion is rough for attachment of ligaments. Caudad is the narrow longitudinal triangular area of the lateral faces of the fused transverse processes of the second and third sacral vertebræ.
Caudal Vertebræ. Vertebræ caudales
([Fig. 1], y, and [Figs. 8] and [9]).—The caudal vertebræ (21-23 in number) decrease gradually in size to the last one. Caudad they become longer and more slender and lose the character of vertebræ. They become finally reduced to mere centra,—slender rods of bone knobbed or enlarged at their two ends ([Fig. 8]). The last one is more pointed than the others and bears at its caudal end a small separate conical piece, the rudiment of an additional vertebra.
Fig. 8.
Fig. 9.
Fig. 8.—Caudal Vertebra, from near the caudal end of the tail.
Fig. 9.—Fourth Caudal Vertebra, ventral view. a, transverse processes; b, cranial articular processes; c, hæmal processes; d, chevron bone.
The parts of a typical vertebra—vertebral arch, transverse processes, cranial and caudal articular processes—may be recognized in the vertebræ as far back as the eighth or ninth. The transverse processes ([Fig. 9], a) are directed caudad and decrease rapidly in length. They are very small on the ninth vertebra, but may be recognized for a considerable distance back of this. The spinous process disappears at about the fourth caudal vertebra, and the vertebral canal becomes gradually smaller caudad, until on the eighth or ninth vertebra it becomes merely a groove open dorsad.