692 NERVOUS SYSTEM OF THE HORSE 



under cover of the niastoido-humeralis. It then passes obHquely through the 

 latter muscle and continues backward on the splenius and 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. 436) is smaller. It enters the sterno- 

 cephalicus muscle l)ehind the cervical angle of the parotid gland. 



The fibers of the spinal part of the aceessory arise from the ventro-lateral eells of the ventral 

 gray column of tlie cord as fur 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 TWELFTH OR HYPOGLOSSAL NERVE 

 The hjrpoglossal nerve (N. hypoglossus) is purely motor and innervates the 

 muscles of the tongue (Fig. 437). Its root-fibers arise from the ventral face of the 

 medulla in linear series about 3 to 4 mm. lateral to the posterior half of the pyramid 

 (Fig. 499). The filaments converge to three or four bundles which perforate the 

 dura mater and unite to form the trunk. The latter emerges through the hypoglos- 

 sal foramen (Fig. 515) 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 tenth and eleventh 

 nerves, turns downward and forward, crosses the external 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 external 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 ramifies on the genio- 

 glossus and supplies the remaining muscles. Anastomoses occur with branches of 

 the lingual nerve. 



In the first part of its course the nerve communicates with the superior 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 

 multipolar cells situated chicflj^ 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 

 connections include: (a) communications by the medial longitudinal fasciculus with the nuclei 

 of termination of otlier cranial nerves; (6) corti co-nuclear fibers which come from the cortex 

 l)y way of the internal capsule and the pyramids and go largely to the nucleus of the opposite 

 side; (c) filx'rs whicli join the dorsal longitudinal bundle of Schlxtz, 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 th(^ horse. They are designated according to their relations to 

 the vertebral colunm as cervical (S), thoracic (18), lumbar (6), sacral (5), and 

 coccygeal (5). Each nerve is connected with the spinal cord l)y two roots, dorsal 

 and ventral (Fig. 497). 



The dorsal (or sujierior) root (Radix dorsalis) is the larger of the two. Its 

 fil)ers (Fihi I'adiciilaria) 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 

 liundle, 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. 



