Part 208

2. The =superior= or =anterior laryngeal nerve= (N. laryngeus cranialis) is larger than the preceding and arises a little behind it (Fig. 437). It crosses the deep face of the origin of the external carotid artery, runs downward and forward over the lateral wall of the pharynx behind the hypoglossal nerve, and passes through the foramen below the anterior cornu of the thyroid cartilage. Its terminal branches ramify in the mucous membrane of the larynx, the floor of the pharynx, and the entrance to the œsophagus; they anastomose with those of the recurrent. At its origin the nerve gives off its small =external branch= (R. externus); this descends to the crico-thyroid muscle, which it supplies, and sends filaments to the crico-pharyngeus also. It may arise from the trunk of the vagus or from the pharyngeal branch.

At the point of origin of the superior laryngeal nerve there is a plexiform widening which is regarded by some authors as the homologue of the ganglion nodosum of man. From it a filament arises which, after a short course, rejoins the vagus or enters the sympathetic trunk. Stimulation of its central end causes a reduction of the blood-pressure, and it is therefore termed the depressor nerve (N. depressor).

3. The =recurrent nerve= (N. recurrens), also termed the inferior or posterior laryngeal nerve, differs on the two sides in its point of origin and in the first part of its course. The =right nerve= (Fig. 429) is given off opposite the second rib, turns around the dorso-cervical artery from without inward, runs forward on the lower part of the lateral surface of the trachea, and ascends in the neck on the ventral face of the common carotid artery. The =left nerve= (Fig. 428) arises from the vagus where the latter begins to cross the aortic arch. It passes back over the ligamentum arteriosum, winds around the concavity of the aortic arch from without inward, runs forward on the lower part of the left face of the trachea, and continues in the neck in a similar position to the right nerve.

It is worthy of note that the left nerve passes beneath the bronchial lymph glands as it winds around the aorta; also that in the next part of its course it lies between the left surface of the trachea and the deep face of the aorta, and is then related to lymph glands which lie along the ventral aspect of the trachea. The left recurrent is often incorporated in part of its course in the anterior mediastinum with a deep cardiac nerve. Further, the left nerve lies at first ventral to, and then upon, the œsophagus in the neck. The right recurrent is given off from or in common with a considerable trunk which connects the vagus with the first thoracic ganglion of the sympathetic. The arrangement here is commonly more or less plexiform, and from it one or two cardiac nerves arise.

The terminal part of each nerve (Fig. 517) lies on the dorsal surface of the trachea, in relation to the œsophagus internally and the carotid artery above. It passes between the crico-arytenoideus posterior and the crico-pharyngeus; the =terminal branches= supply all the muscles of the larynx except the crico-thyroid, and communicate with branches of the superior laryngeal nerve. =Collateral branches= are given off to the cardiac plexus (Rr. cardiaci), to the trachea (Rr. tracheales), to the œsophagus (Rr. œsophagei), and to the inferior cervical ganglion of the sympathetic.

4. =Cardiac branches= (Rr. cardiaci), usually two or three in number, are given off from each vagus within the thorax (Figs. 428, 429). These concur with the cardiac branches of the sympathetic and recurrent nerves to form the cardiac plexus, which innervates the heart and great vessels.

5. Small =tracheal= and =œsophageal branches= (Rr. tracheales et œsophagei) are given off from both vagi in the thorax. These concur with branches from the recurrent nerves and the inferior cervical and anterior thoracic ganglia of the sympathetic in forming the =posterior tracheal= and =œsophageal plexuses=, from which twigs go to the trachea, œsophagus, heart, and large vessels.

6. =Bronchial branches= (Rr. bronchiales) are detached at the roots of the lungs and unite with sympathetic filaments in forming the =pulmonary plexuses=. From the latter numerous branches proceed in a plexiform manner along the bronchi and vessels into the substance of the lungs.

The vagus and glosso-pharyngeal nerves are so closely associated in origin and central connections that they may be described together in this respect.

The =sensory fibers= arise from the petrous and jugular ganglia, and their central parts enter the lateral aspect of the medulla and divide into anterior and posterior branches like the fibers of the dorsal roots of the spinal nerves. Most of the fibers end in arborizations about the cells of the vago-glosso-pharyngeal nucleus of termination, which consists of two parts. Of these the =dorsal sensory nucleus= (Nucleus alæ cinereæ) is situated in the posterior part of the floor of the fourth ventricle and in the adjacent part of the closed portion of the medulla near the median plane. The other part is termed the =nucleus of the solitary tract=, and is so named because its cells are grouped about the bundle (Tractus solitarius) formed by the posterior divisions of the afferent nerve-fibers. It ends about the level of the pyramidal decussation. The secondary central connections are similar to those of the sensory part of the trigeminus.

The =motor fibers= (and those of the medullary part of the accessory) arise from the =dorsal motor nucleus= and the =ventral motor nucleus=. The cells of the former lie in groups along the ventro-medial side of the dorsal sensory nucleus. The latter, also termed the nucleus ambiguus, is situated more deeply in the lateral part of the formatio reticularis.

THE ELEVENTH OR SPINAL ACCESSORY NERVE

The =spinal accessory nerve= (N. accessorius) is purely motor. It consists of two parts which differ in origin and function.

The =medullary part= arises from the lateral aspect of the medulla by several rootlets which are behind and in series with those of the vagus (Fig. 499). The =spinal part= arises from the cervical part of the spinal cord by a series of fasciculi which emerge between the dorsal and ventral roots. The bundles unite to form a trunk which is very small at its origin at the fifth segment of the cord, but increases in size when traced toward the brain, since it continually receives accessions of fibers. It passes through the foramen magnum and joins the medullary part. The trunk thus formed sends its medullary fibers to the tenth and ninth nerves and emerges through the foramen lacerum posterius. It then runs backward and downward with the vagus in a fold of the guttural pouch, separates from that nerve, crosses the deep face of the submaxillary gland and the occipital artery, and divides in the recessus atlantis into dorsal and ventral branches.

FIG. 518.—NECK OF HORSE, AFTER REMOVAL OF PART OF PANNICULUS AND TRAPEZIUS.

a, a′, Mastoido-humeralis; b, anterior superficial pectoral muscle; c, cervical panniculus; d, sterno-cephalicus; e, omo-hyoideus; f, sterno-thyro-hyoideus; g, trachea; h, h′, h″, tendons of splenius, mastoido-humeralis, and trachelo-mastoideus; i, trapezius cervicalis; k, supraspinatus; l, anterior deep pectoral muscle; m, rhomboideus cervicalis; n, serratus cervicis; o, splenius, upper and lower borders of which are indicated by dotted lines; p, parotid gland; q, parotido-auricularis muscle; r, wing of atlas; s, spine of scapula; 1, external maxillary vein; 2, 3, jugular vein; 4, carotid artery; 5, descending branch of inferior cervical artery; 6, cephalic vein; 7–11, ventral branches of second to seventh cervical nerves; 12, cutaneous branch of second cervical nerve; 13, cervical branch of facial nerve; 14, terminal branches of dorsal divisions of cervical nerves; 15, dorsal branch of spinal accessory nerve. (After Ellenberger-Baum, Top. Anat. d. Pferdes.) ]

It is connected by anastomotic branches with the vagus and hypoglossal nerves and the superior cervical ganglion of the sympathetic, and contributes a branch to the pharyngeal plexus.

The =dorsal branch= (R. dorsalis) (Figs. 433, 517, 518) receives a twig from the second and third cervical nerves and turns around the atlantal tendon of the splenius under cover of the mastoido-humeralis. It then passes obliquely 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 behind the cervical angle of the parotid 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 TWELFTH OR HYPOGLOSSAL NERVE

The =hypoglossal 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 hypoglossal 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 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. 497).

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.