In the mid-dorsal line is the series of =spinous processes=, which are
low ridges in the cervical region with the exception of the second and
seventh, reach their maximum height at the fourth and fifth thoracic
vertebræ, and diminish to the fifteenth or sixteenth thoracic. Behind
this they are about equal in height as far as the last lumbar and first
sacral, which are somewhat lower. The second sacral spine is about as
high as the middle lumbar; behind this they diminish rather rapidly in
height and fade out about the third coccygeal. Their =inclination=
backward is most decided at the second thoracic, diminishes from the
sixth or seventh to the fifteenth or sixteenth, which is vertical and is
termed the =anticlinal= or =diaphragmatic vertebra.= Behind this they
are inclined a little forward until the sacrum is reached; here there is
an abrupt change to the backward inclination, so that a considerable
interspinous angle is formed.
On either side of the spinous processes is a =vertebral groove= which
contains the deep muscles of the spine. The floor of the groove is
formed by the laminæ and articular processes. It is wide in the neck and
narrows progressively in the back.
Viewed from the side, the column presents a series of curves. When the
head and neck are in the ordinary neutral position, the anterior part of
the cervical spine forms a gentle curve, concave ventrally. The
posterior cervical and first thoracic vertebræ form a more pronounced
curve in the opposite direction. At the junction of the cervical and
thoracic regions there is a marked change of direction, forming a
ventral projection or angle. At the second thoracic vertebra a gentle
curve, concave ventrally, begins. This is continued to the lumbo-sacral
junction, where there is a change of direction, and hence a promontory.
The sacrum has a variable, but never very pronounced, ventral concave
curvature, which is continued in a much accentuated form in the
coccygeal region. It may be remarked that a line through the summits of
the spines does not correspond to these curves formed by the bodies.
The =vertebral canal=, of course, corresponds in curvature to the
bodies. Its =caliber= varies greatly at different points. The greatest
diameter (ca. 5 cm.) is in the atlas, where it contains the dens of the
axis in addition to the spinal cord, and provision must be made for
extensive movement. It is very much smaller in the axis (ca. 2.5 cm.
wide, 3 cm. high). It widens considerably at the junction of the
cervical and thoracic regions to accommodate the cervical enlargement of
the spinal cord. Beyond this it diminishes, and is smaller in the middle
of the back than at any preceding point; this is correlated with the
small size of the spinal cord and the very limited movement of the
spine. At the middle of the lumbar region it again widens considerably
to contain the lumbar enlargement of the spinal cord. The caliber
diminishes very rapidly from the second sacral segment backward, and the
canal ceases to be complete at the fourth coccygeal vertebra.
The =articular processes= are very large and wide apart in the neck,
greatly reduced and much closer together in the back, larger and
interlocking in the lumbar region.
The =transverse processes= are large and outstanding in the neck, where
they form the outer boundary of a ventral groove occupied by the longus
colli muscle. In the back they are short and stout, and are
characterized by the facets for the tubercles of the ribs. On the first
thoracic vertebra this facet is large, deeply concave, and situated
almost directly outward from the cavity for the head of the rib; traced
backward it becomes smaller and flatter, and gradually comes to lie
behind the cavity for the head of the rib, with which it is fused on the
last and often also on the next to the last thoracic vertebra. The
processes in the lumbar region have a characteristic elongated
plate-like form. In the sacral region they are fused to form the wings
and lateral parts of the sacrum. In the coccygeal region they are at
first of considerable size relatively, but undergo rapid reduction, and
disappear at the fifth or sixth vertebra.
The cavities for the heads of the ribs diminish progressively in size
and depth from first to last.
The =mammillary processes= are usually distinct on the fourteenth to the
seventeenth thoracic vertebræ. In front of these they blend with the
transverse, behind with the anterior articular processes.
The length of the vertebral column (including the intervertebral
fibro-cartilages) in a horse of medium size is about 260 to 265 cm.
(ca. 8 feet 8 inches to 8 feet 10 inches). The relative lengths of the
various regions appear to vary most in the neck and back.
The table below gives the measurements in centimeters in a trotting
stallion of medium size and in an adult Percheron stallion. The
percentages are in round numbers.
─────────┬───────────┬───────────
│ TROTTER │ PERCHERON
─────────┼─────┬─────┼─────┬─────
Cervical │ 60.0│22.7%│ 74.0│25.4%
Thoracic │ 98.0│37.2%│100.0│34.4%
Lumbar │ 31.5│12.0%│ 36.5│12.5%
Sacral │ 20.0│ 7.6%│ 22.5│ 7.7%
Coccygeal│ 54.0│20.5%│ 58.0│20.0%
─────────┼─────┼─────┼─────┼─────
│263.5│100.0│291.0│100.0
─────────┴─────┴─────┴─────┴─────