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 brachiocephahcus. 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 4 mm. 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 cm.). 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 comu 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 hjrpoglossal 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; (6) 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 Sohiitz, a tract which underhes 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 



