812 NERVOUS SYSTEM OF THE HORSE 
The dorsal root is sensory or afferent, 7. 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. 
THE CERVICAL NERVES 
The cervical nerves (Nervi cervicales) (Figs. 556, 558, 650, 655) number eight 
pairs. The first of these emerges through the intervertebral foramen of the atlas, 
the second through that of the axis, and the eighth between the last cervical and 
the first thoracic vertebree. 
The dorsal branches are distributed to the dorso-lateral muscles and skin of 
the neck. They divide usually into lateral and medial branches. The medial 
branches (Rami mediales) run in general across the multifidus and the lamellar part 
of the ligamentum nuche to the skin of the dorsal border of the neck; they supply 
the deep lateral muscles and the skin. The lateral branches (Rami laterales) are 
chiefly muscular in their distribution. The dorsal branches of the third to the 
sixth nerves are connected by anastomotic branches to form the dorsal cervical 
plexus. 
The ventral branches are smaller than the dorsal ones—an exception to the 
general rule. They increase in size from first to last. They supply in general the 
muscles and skin over the lateral and ventral aspect of the vertebrae, but the last 
three enter into the formation of the brachial plexus, and the two or three preceding 
the last give off the roots of the phrenic nerve. An irregular ventral cervical plexus 
is formed by anastomoses established between the ventral branches. The following 
special features may be noted: 
The first cervical nerve emerges through the intervertebral foramen of the 
atlas. Its dorsal branch (N. occipitalis) passes dorso-laterally between the obliqui 
capitis and the recti capitis dorsales and supplies branches to these muscles, the 
scutularis and posterior auricular muscles, and the skin of the poll. The ventral 
branch descends through the alar foramen of the atlas, crosses over the ventral 
straight muscles and the carotid artery under cover of the parotid gland, and divides 
into two branches. The anterior branch enters the omo-hyoideus muscle. The 
posterior branch passes downward and backward under cover of that muscle, unites 
with a branch of the ventral division of the second cervical nerve, and continues its 
course on the ventro-lateral surface of the trachea to enter the sterno-thyro-hyoideus 
behind the intermediate tendon. In the recessus atlantis the ventral branch is 
connected by one or more twigs with the anterior cervical ganglion of the sym- 
pathetic, and a little lower with the hypoglossal nerve. It also sends branches to 
the ventral straight muscles of the head and the thyro-hyoideus. Below the atlas 
the ventral branch is crossed superficially by the spinal accessory nerve, the occi- 
pital artery, and the ventral cerebral vein. 
The second cervical nerve is larger than the first. It emerges from the verte- 
bral eanal through the intervertebral foramen of the anterior part of the arch of the 
axis. Its dorsal branch ascends between the complexus and the ligamentum 
nuch and ramifies in the skin of the poll. The ventral branch gives off muscular 
branches to the rectus capitis ventralis major, and anastomotic branches to the 
spinal accessory and the ventral divisions of the first and third cervical nerves; 
one of these crosses over the carotid artery and concurs in the formation of the nerve 
to the sterno-thyro-hyoideus mentioned above. The ventral branch then becomes 
superficial by passing between the two parts of the brachiocephalicus, and divides 
into posterior auricular and cutaneous branches. The posterior auricular nerve 
