THE LUMBAR VERTEBRA 39 



has on its ventral aspect a large concave facet for articulation with the tubercle of 

 the first rib. The spinous process is curved backward and tapers to a point. Its 

 length IS usually about three or four inches (ca. 8 to 10 cm.). This vertebra may be 

 mistaken at first glance for the last cervical, but is promptly identified by the three 

 costal facets on each side and the length of the spine. 



The last thoracic vertebra is distinguished by the absence of the posterior pair 

 of costal facets, and the confluence of the anterior pair with those on the transverse 

 processes. 



The serial position of others may be determined at least approximately by the 

 following data: (1) The bodies gradually diminish in length and width to the 

 middle of the region and then increase slightly. Their costal facets become smaller 

 and less concave from first to last. The ventral crest is distinct on three or four 

 vertebra} at either end of the region. (2) The transverse processes diminish in 

 size and are placed lower down as they are traced backward. Their costal facets 

 become smaller and lower in position; on the last (and sometimes on its predecessor 

 also) it fuses with the costal facet of the body. The upper non-articular part of 

 the process gradually becomes more sharply defined, and in the last four or five 

 forms a distinct mammillary process. (3) The spinous processes increase in length 

 to the third and fourth, and then gradually diminish to the fifteenth, beyond which 

 they have about the same length. The backward inclination is most pronounced 

 in the second, the sixteenth is vertical, and the last two are directed a little forward. 

 The longest spines (i. e., those of the withers) are the thickest and have expanded 

 summits which remain more or less cartilaginous; the others are more plate-like, 

 and are surmounted by a thick lip. The second spine is more than twice as large 

 as the first. The summits of the fourth and fifth usually form the highest point 

 of the withers. 



Development. — There are six or seven centers of ossification, 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 VERTEBRE 



The lumbar vertebrae (Vertebrae 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 semi-elliptical on cross-section, and present a 

 distinct ventral crest. From the fourth backward they become wider and flatter 

 and the ventral crest subsides. 



The arches of the first three are about equal in size and similar to that of the 

 last thoracic; behind this they increase in breadth and height. The posterior 

 notches are much deeper than the anterior ones. 



The anterior articular processes are fused with the mammillary processes, and 

 present dorsally 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 concave surfaces of the anterior pair of the next vertebra. 



The transverse processes are elongated plates, flattened dorso-ventrally, which 

 project outward and usually curve slightly downward; their length increases to 

 the third or 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 medial part of the posterior 

 border for articulation with the sixth process; the latter has a corresponding con- 

 vex 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 medial part of the sixth process 



