The =base= is slightly oblique and presents a smooth transversely oval articular facet (the cranial end of the centrum of the first sacral vertebra), for articulation with the centrum of the last lumbar vertebra. Dorsad of this is the sacral canal, more depressed than the vertebral arch craniad of it. It supports a spinous process (Fig. 6, _a_) which is directed dorsad. At the junction of its lamina and radix is seen the prominent cranial articular process (_b_) with sometimes slight indications of a mammillary process on its lateral surface. Laterad of the articular facet is seen the cranial face of the expanded “pseudo-transverse process” (_c_) of the first sacral vertebra. The ventral border of the base is concave ventrad, forming an arc of about 120 degrees. The =apex= shows the caudal end of the last sacral centrum. Dorsad of this are the vertebral arch with a very short spinous process (_a′_), and the caudal articular processes (_d_). Laterad of the centrum appears the laterally directed thin transverse process (_e_). [Illustration: FIG. 6.--SACRUM, DORSAL SURFACE. FIG. 7.--SACRUM, VENTRAL SURFACE. Fig. 6.--1, 2, 3, the three sacral vertebræ. _a_, _a′_, spinous processes; _b_, cranial articular process of first sacral vertebra; _c_, expanded transverse process of first sacral vertebra; _d_, caudal articular processes of third sacral vertebra; _e_, transverse processes of third sacral vertebra; _f_, tubercles formed by fused articular processes of the vertebræ; _g_, dorsal (or posterior) sacral foramina. Fig. 7.--1, 2, 3, the three sacral vertebræ. _a_, the transverse ridges formed by the union of the centra; _b_, cranial articular processes of first vertebra; _c_, transverse process of first vertebra; _d_, caudal articular processes of third vertebra; _e_, transverse processes of third sacral vertebra; _f_, _f′_, ventral (or anterior) sacral foramina; _g_, notch which helps to form third ventral sacral foramen.] The ventral or pelvic surface (Fig. 7) is smooth, concave craniad, convex caudad, and crossed by two transverse ridges (_a_) along which are seen the ossified remains of the intervertebral fibro-cartilages. At the ends of the first ridge is a pair of nearly circular =ventral= (or anterior) =sacral foramina= (_f_) for the passage of sacral nerves. At the end of the second ridge is a pair of ventral sacral foramina (_f′_), smaller than the first pair and continued laterocaudad into shallow grooves for the ventral rami of the sacral nerves. That portion of the bone lying laterad of a line joining the medial borders of these two pairs of foramina is known as the =lateral mass= of the sacrum and is composed of the fused transverse processes of the sacral vertebræ. At the caudal margin of the ventral surface there is a notch between the lateral mass and the centrum (_g_). When the caudal vertebræ are articulated, this notch helps to form a foramen for the third sacral nerve.