The Twelfth or Hypoglossal Nerve

The hypoglossal nerve (N. hypoglossus) is purely motor and innervates the muscles of the tongue (Fig. ). 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. ). 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. ) 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 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 connections include: (a) communications by the medial longitudinal fasciculus with the nuclei of termination 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 Schütz, 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. ).

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. ) 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.

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.