Page 95 - Adams and Stashak's Lameness in Horses, 7th Edition

Functional Anatomy of the Equine Musculoskeletal System 61

aspect of the body and the medial and ventral parts of caudal. Cervical vertebrae three, four, and five bear a
the arch is the vertebral foramen; where vertebral distinct ventral crest on their bodies. This crest is dimin­
VetBooks.ir spinal cord, the resulting passageway is called the verte­ cervical vertebra. Articular processes on these cervical
ished in size on the sixth and absent from the seventh
foramina of adjacent vertebrae are aligned to admit the
vertebrae are large, with prominent oval fovea for artic­
bral canal. The vertebral canal is widest in the caudal
cervical–cranial thoracic region, where it accommodates ulation between vertebral arches. The cranial articular
the cervical enlargement of the spinal cord. A second processes present their foveae in the dorsomedial direc­
dilation of the canal occurs in the lumbar region where tion; caudal articular processes have complementary
the lumbosacral enlargement of the cord resides. foveae directed in the ventrolateral direction. Transverse
The vertebral arch comprises the pedicles and lami­ processes are broad, each with two thick tubercles for
nae, which together create the “roof” over the spinal muscular attachment. Transverse processes of the sixth
cord. The pedicles are the vertical attachments to the cervical vertebra are especially robust, while those of the
vertebral body. The dorsal part of the arch is created by seventh are somewhat diminished relative to the other
the right and left laminae. The arch is characterized by cervical vertebrae. The dorsocaudal aspect of the sev­
vertebral notches, indentations on the cranial and cau­ enth cervical body features a costal fovea that partici­
dal aspects of the pedicle. When individual vertebrae are pates in the synovial articulation of the head of the first
articulated, the cranial vertebral notch of one vertebra rib with the seventh cervical and first thoracic vertebrae.
abuts the caudal vertebral notch of another, creating an The spinous process of the seventh cervical vertebra is
intervertebral foramen through which the spinal nerve tall compared with other cervical vertebrae.
emerges from the vertebral canal.
The vertebral arch features other bony processes that
bear synovial joints between adjacent vertebrae and that Thoracic Vertebrae
serve as sites of attachment for epaxial muscles. Each There are usually 18 thoracic vertebrae in the horse,
vertebra has a single dorsal midline spinous process and although on occasion there may be one more or one less
two transverse processes that arise near the point at than typical. The bodies of the thoracic vertebrae tend to
which the pedicle attaches to the body. The dorsal con­ be short with a small vertebral arch dorsally. The spinous
tour of the equine thorax and loin is largely determined processes are relatively tall, with the first four or five
by the relative size and prominence of the spinous pro­ increasing in height and more caudal spinous processes
cesses of thoracic and lumbar vertebrae. Arising adja­ gradually decreasing in height until at the level of the
cent to the spinous process are a pair of cranial articular 12th thoracic vertebra, after which they are the same
processes and a pair of caudal articular processes. height as those of the lumbar vertebrae. The tall spinous
processes of those first 12 vertebrae constitute the with­
ers. The dorsal apex of the spines is somewhat expanded
Cervical Vertebrae
and in young horses surmounted by cartilage. The carti­
The first two cervical vertebrae are highly modified to lage is replaced by bone as the horse ages, with the car­
meet their specialized function in permitting movement tilages associated with the prominence of the withers
of the head. The first vertebra is the atlas. It lacks the persisting the longest at 10 years or more. The anticlinal
cylindrical body characteristic of other vertebrae, instead vertebra is defined as the one whose spinous process is
taking the form of a bony ring comprising dorsal and perpendicular to the long axis of the vertebral column.
ventral arches. The spinous process is likewise absent. The spinous processes of more cranial vertebrae incline
The transverse processes are modified into the wings of caudad, while those of more caudal vertebrae incline
the atlas. These are robust, bent in a ventrolateral direc­ craniad. In the horse, the anticlinal vertebra is usually
tion, and strongly concave ventrally. Their craniolateral the 16th and occasionally the 14th.
edges form a prominent palpable ridge caudal to the The vertebral bodies possess cranial and caudal cos­
ramus of the mandible. The dorsal aspect of the wing of tal foveae for articulation with the heads of ribs, except­
the atlas bears three foramina: the transverse foramen, ing the last thoracic vertebra, which features only cranial
the alar foramen, and the lateral vertebral foramen. The costal foveae. Transverse processes are irregular, largest
cranial aspect of the atlas possesses two deeply concave in the cranial thoracic vertebrae and gradually decreas­
cranial articular foveae, which form a synovial joint ing in size toward the lumbar region. Mammillary pro­
(the atlanto‐occipital joint) with the occipital condyles. cesses appear in the caudal thoracic region. These are
The caudal articular foveae are also concave and partici­ directed craniad and arise primarily from the transverse
pate in the synovial atlantoaxial joint. processes. In the most caudal of the thoracic vertebrae,
The second cervical vertebra is the axis. The body of they arise in common from the transverse and cranial
the axis is long, and its cranial extremity is modified into articular processes and may for this reason be called
a scoop‐like projection called the dens, which features a mamilloarticular processes.
rounded ventral articular surface that articulates with
the floor of the atlas. The caudal extremity of the axis’
body is deeply concave where it articulates with the body Lumbar Vertebrae
of the third cervical vertebra. The spinous process of the There are usually six lumbar vertebrae, although five
axis is tall and long, modified into a thick midline plate. and seven have also been reported. The cylindrical bod­
The transverse processes are small and caudally directed. ies of the lumbar vertebrae are somewhat flattened dor­
The third through seventh vertebrae are similar to soventrally, especially the last three. Except for the
one another and follow the basic pattern of most verte­ seventh and sometimes the sixth lumbar vertebrae, a
brae. They are progressively shorter from cranial to ventral crest is prominent. The spinous processes project