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.)
]
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.