THE LUMBAR VERTEBRX 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 
vertebre 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 (7. 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 VERTEBRZE 
The lumbar vertebre (Vertebre 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 
