=Development.=—There are =six= or =seven centers=, three for the body, two for the arch, and one for the spinous process; some of the latter have an additional center for the summit. THE LUMBAR VERTEBRÆ The =lumbar vertebræ= (Vertebræ lumbales) are usually six in number in the horse. They are characterized by the size and form of their transverse processes. The =bodies= of the first three are triangular on cross-section, and present a distinct ventral crest. From the fourth backward they become wider and flatter and the ventral crest fades out. [Illustration: FIG. 16.—FIRST THORACIC VERTEBRA AND UPPER PART OF FIRST RIB OF HORSE, ANTERIOR VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ] The =arches= of the first two or three are about equal in size and similar to that of the last thoracic; from the fourth they decrease noticeably in breadth and height. [Illustration: FIG. 17.—LAST TWO THORACIC AND FIRST LUMBAR VERTEBRÆ OF HORSE, LATERAL VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ] The =anterior articular processes= are fused with the mammillary processes, and present superiorly concave surfaces for articulation with the posterior pair of the preceding vertebra. The =posterior articular processes= project distinctly from the arch at the base of the spinous process, and have ventrally convex articular surfaces, which fit into the grooved surfaces of the anterior pair of the next vertebra. The =transverse processes= are large plates, flattened dorso-ventrally, which project outward and usually curve slightly downward; their length increases to the third and fourth, and then diminishes to the last, which is the shortest. The first one or two usually curve somewhat backward, the last two decidedly forward. Those of the fifth have an oval concave facet on the inner part of the posterior border for articulation with the sixth process; the latter has a corresponding convex facet on the anterior border, and a larger concave surface on the posterior border for articulation with the wing of the sacrum. Sometimes the fifth process has a small surface for articulation with the fourth. The inner part of the sixth process is thick, the outer part thinner, narrower, and curved forward. The inner part of the fifth is also somewhat thickened. The =spinous processes= resemble those of the last two thoracic vertebræ. They are usually about equal in height, but minor differences are common, and the width diminishes in the last three. =Development.=—This is similar to that of the thoracic vertebræ. The extremities of the transverse processes remain cartilaginous for some time after ossification is otherwise complete. [Illustration: FIG. 18.—SECOND LUMBAR VERTEBRA OF HORSE, ANTERIOR VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]