44 THE SKELETON OF THE HORSE 
Variations.—The number is said by good observers to vary between fourteen and twenty- 
one. In old age the first is often fused with the sacrum, and sometimes with the second. The 
arch of the third may be open. 
THE VERTEBRAL COLUMN AS A WHOLE 
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 thoracie vertebrie, 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 coceygeal. Their inclination back- 
ward is most decided at the second thoracic and diminishes from the sixth or seventh 
to the sixteenth thoracic, which is vertical and is termed the anticlinal or dia- 
phragmatic 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 arches 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 
vertebra: 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, con- 
cave 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 coceygeal region. It should be noted that a line 
through the summits of the spines does not correspond to these curves formed by 
the bodies. 
The vertebral canal corresponds in its curvature to that of the bodies. Its 
caliber varies greatly at different points. The greatest diameter 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. It 
widens considerably at the junction of the cervical and thoracic regions to accom- 
modate 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 corre- 
lated with the small size of the spinal cord and the very limited movement of the 
spine here. Beyond the middle of the lumbar region it again enlarges 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 com- 
plete at the fourth coccygeal vertebra. 
The transverse and vertical diameters of the vertebral canal at various points are given 
in the annexed table. The measurements were made on a horse of medium size; they represent 
the maximum width and height in the middle of the vertebra and are expressed in centimeters. 
VERTEBRA | cl C2 C4: 1 Pez | ri0l || Mers L6 81 | $5 
—— ~ | _- — | | | - | - —— | 
Transverse....... oe 5.2 2.6 2.6 3.5 2t3 2.4 4.0 4.0 18 
Vertical... eS nee 4.2 2.5 2a 25 LT, LS 2.5 on: 15 
The articular processes are very large and wide apart in the neck, greatly 
