The =spinal nerves= (Nervi spinales) are arranged in pairs, of which
there are usually forty-two in the horse. They are designated according
to their relations to the vertebral column as =cervical= (8), =thoracic=
(18), =lumbar= (6), =sacral= (5), and =coccygeal= (5). Each nerve is
connected with the spinal cord by two roots, dorsal and ventral (Fig.
497).
The =dorsal= (or superior) =root= (Radix dorsalis) is the larger of the
two. Its fibers (Fila radicularia) spread out in fan shape and join the
cord in a linear series along the dorso-lateral groove. Laterally the
fibers converge to form a compact bundle, on which is a gray nodular
enlargement, the =spinal ganglion= (Ganglion spinale). Beyond the
ganglion the dorsal root joins the ventral root to constitute the nerve.
The ganglia are external to the dura mater, and are situated in the
intervertebral foramina, except in the case of the sacral and coccygeal
nerves, the ganglia of which lie within the vertebral canal. Those of
the coccygeal nerves are intradural.
The ganglia vary greatly in size; that of the first cervical nerve is
scarcely as large as a hemp seed, while that of the eighth cervical is
about 2 cm. long and 1 cm. wide. On the large roots connected with the
cervical and lumbar enlargement of the cord there are multiple ganglia
of varying sizes interposed in the course of the root-bundles. The
fibers of the dorsal roots arise from the cells of the spinal ganglia;
each cell gives off a fiber which enters the spinal cord and another
which passes into the nerve.
The =ventral= (or inferior) =root= (Radix ventralis) contains fewer
fibers than the dorsal root, except in the case of the first cervical
nerve. It arises from the ventral surface of the spinal cord (Fig. 495)
by means of numerous small bundles of fibers which do not form a linear
series but emerge from the cord over an area three to five millimeters
in width (ventral root zone). The fibers are processes of the large
cells of the ventral gray columns of the spinal cord. There is no
ganglion on the ventral root.
In the cervical, thoracic, and anterior lumbar regions the bundles of
both roots pass through separate openings in linear series in the dura
mater before uniting into a root proper. Further back the bundles of
each root unite within the dura. In the anterior part of the cervical
region and in the thoracic part of the cord there are intervals of
varying length between adjacent roots, but in some places the fibers
of adjacent roots overlap and an exchange of fibers may be observed.
Many of the roots are directed almost straight outward or incline
slightly backward, but the posterior lumbar, sacral, and coccygeal
roots and nerves run backward to reach the foramina through which they
emerge. The distance thus to be traversed increases from before
backward, so that these nerves form a tapering sheaf around the conus
medullaris and filum terminale in the last lumbar vertebra and the
sacrum which is known as the cauda equina.
[Illustration:
FIG. 519.—VERTEBRAL CANAL OPENED BY SAWING OFF THE ARCHES. (After
Schmaltz. Atlas d. Anat. a. Pferdes.)
]
The =size= of the spinal nerves varies greatly. The largest are
connected with the cervical and lumbar enlargements.
In or immediately after its emergence from the intervertebral foramen
each spinal nerve gives off a small =meningeal= or =recurrent branch=
(Ramus meningeus). This is joined by a bundle of fibers from the ramus
communicans and enters the vertebral canal, in which it is distributed.
Each nerve then divides into two primary branches, =dorsal= and
=ventral= (Ramus dorsalis, ventralis). The =dorsal branches= are smaller
than the ventral, except in the cervical region. They are distributed
chiefly to the muscles and skin of the dorsal part of the body. The
=ventral branches= supply in general the muscles and skin of the ventral
parts of the body, including the limbs. Each nerve or its ventral branch
is connected with an adjacent ganglion of the sympathetic system by at
least one small short branch known as a =ramus communicans=. Many nerves
have two and some have three such rami. A nerve may be connected with
two ganglia, and a ganglion may be connected with two nerves.
The dorsal root is sensory or afferent, _i. e._, it conveys impulses
to the central system. Its fibers are axones of the cells of the
spinal ganglion. The ventral root is motor or efferent, and conveys
impulses toward the periphery. Its fibers are axones of the large
cells in the ventral gray columns of the spinal cord. The common trunk
or nerve formed by the union of the two roots contains both kinds of
fibers, as do also their primary divisions. In addition to these
fibers, which are distributed to the skeletal muscles and the skin,
the spinal nerves contain fibers derived from the sympathetic system
through the rami communicantes; these go to the glands and unstriped
muscle and are designated secretory and vasomotor fibers.