The transverse processes of this region are considered equivalent to the proper transverse process + the costal element, and hence the distinctive term processus lateralis (s. costarius) has been proposed. The occurrence of a lumbar rib in connection with the transverse process of the first lumbar is not rare. Reduction of the number to five has been observed frequently, and may or may not be compensated by an additional thoracic vertebra. This variation is not more common in certain races as Sanson and others have maintained. Very few cases are recorded of seven lumbar vertebræ—especially with the normal thoracic number. An anomalous vertebra with mixed thoracic and lumbar characters sometimes occurs at the junction of the two regions. THE SACRUM The =sacrum= (Os sacrum) is formed by the fusion of five vertebræ usually, and is conveniently described as a single bone. It is triangular in form and is wedged in between the ilia, with which it articulates very firmly on each side. Its long axis is gently curved, and slightly oblique, so that the posterior end is a little higher than the anterior. It presents two surfaces, two borders, a base, and an apex. The =dorsal surface= presents centrally the five sacral spines, which are directed upward and backward, and have (with the exception of the first) tuberous summits which are sometimes bifid. The first spine is relatively thin and narrow, and is not so high as the internal angle of the ilium. The second is the longest, and the height diminishes rapidly to the last. The bases of the spines are often fused in old subjects. On either side of the spines is a groove, in which are the four =dorsal sacral foramina= (Foramina sacralia dorsalia); the dorsal branches of the sacral nerves emerge through them. The =ventral= or =pelvic surface= (Facies pelvina) is concave in its length, wide in front, narrow behind. It is marked by four more or less distinct =transverse lines= (Linæ transversæ), which indicate the demarcation of the bodies of the vertebræ. At the ends of these lines are the =ventral sacral foramina= (Foramina sacralia ventralia), which are larger than the dorsal series and diminish in size from first to last; they transmit the ventral divisions of the sacral nerves. The dorsal and ventral foramina communicate with the sacral canal and are together equivalent to the usual intervertebral foramina. [Illustration: FIG. 19.—SACRUM OF HORSE, DORSO-LATERAL VIEW. _C_, Body of first sacral vertebra; _A_, arch of first vertebra; _Cs_, sacral canal; _P.s._ 1–5, sacral spines; _P.t._, wings of sacrum; 1, 1′, surfaces for articulation with transverse processes of last lumbar vertebra; _F.a._, auricular surface; 2, 2′, articular processes; 3–6, dorsal sacral foramina; 7, interarcuate space; 8, lateral border; _h_, apex. (Struska, Anat. d. Haustiere.) ]