810 NERVOUS SYSTEM OF THE HORSE 
The dorsal branch (R. dorsalis) (Figs. 558, 649, 650) receives a twig from the 
second and third cervical nerves and turns around the atlantal tendon of the splenius 
under cover of the brachiocephalicus. It then passes obliquely through the latter 
muscle and continues backward on the cervical part of the serratus, inclines upward 
across the anterior deep pectoral and the supraspinatus, and enters the deep face 
of the trapezius, in which it ramifies. 
The ventral branch (R. ventralis) (Fig. 561) is smaller and much shorter. It 
passes downward and backward under cover of the cervical angle of the parotid 
gland and enters the deep face of the sterno-cephalicus muscle at the junction of the 
muscular substance with the tendon of insertion. It usually crosses over the 
carotid artery just behind the lateral lobe of the thyroid gland. 
The fibers of the spinal part of the accessory arise from the ventro-lateral cells of the ventral 
gray column of the cord as far back as the fifth cervical segment. The fibers of the medullary 
part come chiefly from the nucleus ambiguus in common with the motor fibers of the vagus. 
THE HYPOGLOSSAL NERVE 
The hypoglossal nerve (N. hypoglossus) is purely motor; it innervates the 
muscles of the tongue and the genio-hyoideus (Fig. 562). Its root-fibers arise from 
the ventral face of the medulla in linear series about 3 to 4mm. lateral to the poste- 
rior half of the pyramid (Fig. 629). The filaments converge to three or four bundles 
which perforate the dura mater and unite to form the trunk. The latter emerges 
through the hypoglossal foramen (Fig. 647) and runs downward and backward 
between the guttural pouch and the capsule of the atlanto-occipital articulation 
for a distance of a little less than an inch (ca. 2 em.). It then passes between the 
vagus and accessory nerves, turns downward and forward, crosses the lateral face 
of the external carotid artery, and continues over the pharynx parallel with the 
great cornu of the hyoid bone and behind the external maxillary artery. It then 
crosses beneath the artery, runs forward on the lateral face of the hyo-glossus muscle, 
and divides into its terminal branches (Rami linguales). The smaller branch 
supplies the stylo-glossus, hyo-glossus, and lingualis. The larger branch passes 
upward and forward between the hyo-glossus and genio-glossus, ramifies on the latter, 
and supplies the remaining muscles and the genio-hyoideus. Anastomoses occur 
with branches of the lingual nerve. 
In the first part of its course the nerve communicates with the anterior cervical 
ganglion and with the ventral branch of the first cervical nerve, and gives filaments 
to the pharyngeal branch of the vagus and the pharyngeal plexus. 
The fibers of the nerve arise from the hypoglossal nucleus, an elongated group of large multi- 
polar cells situated chiefly under the posterior part of the floor of the fourth ventricle, close to the 
median plane. The two nuclei are connected by commissural fibers. The other central connec- 
tions include: (a) communications by the medial longitudinal fasciculus with the nuclei of ter- 
mination of other cranial nerves; (b) cortico-nuclear fibers which come from the cortex by way of 
the internal capsule and the pyramids and go largely to the nucleus of the opposite side; (c) fibers 
which join the dorsal longitudinal bundle of Schiitz, a tract which underlies the floor of the fourth 
ventricle and is traceable forward below the cerebral aqueduct. 
THE SPINAL NERVES 
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. 628). 
The dorsal 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. The fibers converge laterally to form a compact bundle, on 
