Part 81

VII. N. FACIALIS (Fig. 155).--The seventh or facial nerve arises from the trapezium at the caudal margin of the pons, craniad of the origin of the eighth nerve, as described in the account of the brain (Fig. 138, VII). It passes into the internal auditory meatus, traverses the facial canal through the petrous bone, and emerges at the stylomastoid foramen.

Within the facial canal the nerve bears an enlargement, the =ganglion geniculi=. It gives off within the canal a branch to the stapedius muscle, and the =superficial petrosal= and =chorda tympani= nerves.

The =superficial petrosal= (N. petrosus superficialis major) nerve passes into the hiatus facialis, through the canal which forms its continuation, and joins the sphenopalatine ganglion (p. 373).

a, N. facialis; b, branch to digastric; c, N. auricularis posterior; d, branch to inside of ear; e, ventral ramus; f, dorsal ramus; g, temporal nerve; h, zygomatic nerve; i, nerve to stylohyoid muscle; j, inferior buccal nerve; k, superior buccal nerve; l, infraorbital branches of fifth nerve; m, lachrymal branches of fifth nerve; n, auriculotemporal branches of fifth nerve. 1, M. digastricus; 2, M. stylohyoideus; 3, M. masseter.]

The =chorda tympani= is given off two or three millimeters before the emergence of the facial at the stylomastoid foramen. It passes into the tympanic cavity, extends across it between the malleus and incus, passing close against the tensor tympani muscle, and leaves it (by a small cleft, the canal of Huguier) between the bulla and the squamous portion of the temporal. It then passes craniad (Fig. 154, c), ventrad of the root of the zygomatic process of the temporal, and joins the lingual nerve (Fig. 154, b) (branch of the third division of the fifth nerve).

On emerging from the stylomastoid foramen the facial nerve (Fig. 155, a) gives off at once a small branch (b) to the digastric muscle (1), and a larger branch, =N. auricularis posterior= (c), which passes dorsocaudad about the base of the ear, supplying some of its muscles. Another small branch (d) pierces the cartilaginous external ear and is distributed to its inner surface. The facial nerve then curves craniad about the proximal part of the cartilaginous external ear, and divides five or six millimeters from its emergence into two main branches, which may be designated as the =dorsal= (f) and =ventral= (e) rami. A third small branch may also rise from the point of union of the two; it is usually, however, a twig from the dorsal ramus, and passes to M. submentalis.

The dorsal ramus (f) sends two or three small branches to the cheek (including the one just mentioned), and divides into the =temporal= (g) and the =zygomatic= (h) branches. The former (g) passes along the cranial margin of the external ear, supplying the superficial muscles, and extends into the temporal region, where it lies deeper than the terminal branches of the lachrymal nerve (m) (from the fifth cranial). The =zygomatic= branch (h) passes across the malar bone to the caudal angle of the eye, sends branches into both eyelids, anastomoses with twigs from the lachrymal branch (m) of the fifth nerve, and passes along the medial side of the eye to the lateral surface of the nose, where it ramifies.

The ventral ramus (e) gives off a minute branch (i) to the stylohyoid muscle (2), then proceeds toward the angle of the mouth and divides into =superior= (k) and =inferior= (j) buccal branches, the former to the muscles of the upper lip and the contiguous regions, the latter to those of the lower lip and chin.

The seventh nerve thus supplies most of the muscles of the head except those of mastication, and of these it supplies the digastric.

VIII. N. ACUSTICUS.--The eighth or auditory nerve takes origin (Fig. 138, VIII) from the floor of the fourth ventricle, as before described (page 347). It passes into the internal auditory meatus and is distributed to the internal ear within the petrous bone.

IX. N. GLOSSOPHARYNGEUS (Fig. 156, a).--The ninth or glossopharyngeal nerve takes origin (Fig. 138, IX) from the side of the medulla, as described in the account of the brain. Close to its origin its trunk bears a small enlargement, the =ganglion superius=. It passes along with the tenth and eleventh nerves through the jugular foramen. At its exit it has a ganglionic enlargement, the =ganglion petrosum=; this is connected by fine fibres with the ganglion nodosum (Fig. 156, d) of the vagus. The glossopharyngeal (Fig. 156, a) then passes craniad over the surface of the tympanic bulla (15) and mediad of the digastric muscle. It continues mediad of the carotid artery, and as it approaches the cranial cornu of the hyoid divides into two portions, one of which passes to the muscles and mucosa of the pharynx, and the other to the tongue, where it is the special nerve of taste.

X. N. VAGUS.--The vagus nerve arises from the side of the medulla in the manner described under the Brain (page 346 and Fig. 138, X). It passes through the jugular foramen along with the glossopharyngeal and accessory nerves.

=Cervical Portion of the Vagus= (Fig. 156, d, d′).--In the foramen, or just before entering it, it presents a ganglionic enlargement, the =ganglion jugulare= (or “ganglion of the root”), and a short distance beyond the foramen it forms a second ganglion, the =ganglion nodosum= (d) (or “ganglion of the trunk”), which lies dorsocaudad of the superior cervical sympathetic ganglion (e). The ganglia of the vagus and sympathetic are closely bound together by connective tissue, and that portion of the vagus craniad of the ganglion nodosum is interconnected by a network of nervous fibres with the sympathetic (e), hypoglossal (b), and accessory (c) nerves. From the ganglion nodosum the vagus (d′) passes caudad, closely bound up with the sympathetic; the two lie at the side of the common carotid artery. Just before entering the thorax the two separate, the vagus being the larger and lying ventrad of the sympathetic. (In rare cases the two are distinctly separated throughout their length.)

a, N. glossopharyngeus; b, N. hypoglossus; c, N. accessorius; d, ganglion nodosum of vagus nerve; d′, N. vagus; e, ganglion cervicale superius of sympathetic; f, ramus descendens of N. hypoglossus (united with first cervical); g, branch to M. thyrohyoid from ramus descendens of N. hypoglossus; h, N. laryngeus superior of N. vagus; i, united vagus and sympathetic nerves; j, first cervical nerve; k, second cervical nerve; l, N. pharyngeus from vagus. 1, M. masseter; 2, outline of external ear; 3, M. splenius; 4, M. levator scapulæ ventralis; 5, M. longus capitis; 6, trachea; 7, M. sternothyreoideus; 8, M. sternohyoideus; 9, M. thyreohyoideus; 10, M. constrictor pharyngis inferior; 11, M. constrictor pharyngis medius; 12, cut end of M. stylohyoideus; 13, M. hyoglossus; 14, M. stylopharyngeus; 15, bulla tympani; 16, M. jugulohyoideus; 17, M. styloglossus; 18, M. mylohyoideus.]

Branches of the vagus in the cervical region (Fig. 156).

a. =N. auricularis.=--This leaves the ganglion jugulare, passes into the petrous bone to the facial canal, leaves the skull with the facial nerve by the stylomastoid foramen, and is distributed to the external ear.

b. =N. pharyngeus= (l).--This leaves the vagus craniad of the ganglion nodosum, passes ventrad, sends a small communicating branch to N. laryngeus superior (h), and is distributed to the pharyngeal muscles and cranial part of the œsophagus.

c. =N. laryngeus superior= (h).--This arises from the ganglion nodosum, passes ventrad, crossing the medial surface of the carotid artery, and reaches the larynx at about the level of the caudal end of M. thyreohyoideus. It passes into the larynx and supplies its mucosa.

=Thoracic Portion of the Vagus= (Fig. 157).--As the vagus (b) and sympathetic (c) separate to enter the thorax, the vagus (b) lies more ventrad. The right vagus now lies along the lateral surface of the trachea, the left one (Fig. 157) along that of the œsophagus. At about the level of the first rib or further craniad there arise, apparently from the vagus (but really from the sympathetic), two slender nerves which pass caudad parallel and close to the vagus, lying slightly dorsad of it. These are the =cardiac nerves= (i); they arise from the sympathetic farther craniad in the neck region, accompany the vagus, and seem to branch from it. In cases where vagus and sympathetic are separate throughout their courses, these nerves arise clearly from the sympathetic. The cardiac nerves pass to the heart and aid in forming a network of nerves known as the =cardiac plexus= (k).

The vagus usually receives also, at about the level of the first rib, one or two communicating branches from the middle cervical ganglion (d) of the sympathetic. It then passes to the roots of the lungs, crossing the lateral surface of the aortic arch on the left side, and the medial surface of the azygos vein on the right side. As it crosses the aortic arch the left vagus gives off =N. laryngeus inferior= (j). This curves around the caudal side of the aortic arch to the lateral surface of the trachea (o), then extends on the lateral and ventral surface of the trachea craniad into the neck region. In the neck it passes to the dorsolateral side of the trachea and reaches the larynx. It passes into the larynx between the cricoid and thyroid cartilages, and supplies the muscles of the larynx. It anastomoses with the superior laryngeal.

1-13, the thirteen ribs (cut); V-VIII, the fifth to eighth cervical nerves (cut); I′, first thoracic nerve (cut). a, combined trunk of vagus and sympathetic; b, vagus; c, sympathetic; d, middle cervical ganglion of sympathetic; e, inferior cervical ganglion; e′, communicating branch to sixth, seventh, and eighth cervical nerves; f, phrenic nerve; g, loop of sympathetic about subclavian artery, between the middle and inferior cervical ganglia; h, cardiac branches from the inferior cervical ganglion; i, cardiac branch passing from sympathetic along with vagus to heart; j, inferior laryngeal nerve; k, pulmonary and cardiac plexus; l, ventral œsophageal branch of vagus; l′, similar branch from the opposite side; m, dorsal œsophageal branch of vagus; n, great splanchnic nerve; o, trachea; p, œsophagus; q, aorta; r, heart; s, root of lung.]

The right inferior laryngeal nerve is given off from the right vagus much further craniad than the left; it curves around the right subclavian artery just caudad of the origin of the internal mammary artery, then passes to the trachea, and has a course and distribution like that of the left side.

On reaching the root of the lungs the vagus divides into numerous branches which form the =pulmonary plexus= (k) over the roots of the lungs, and extends onto the basis of the heart as the =cardiac plexus=. From these plexuses numerous branches pass to the heart, lungs, pulmonary artery, pericardium, etc. The cardiac branches from the sympathetic, mentioned above, are also connected with the cardiac plexus.

Caudad of the pulmonary plexus the vagus is continued on each side as two trunks, a =dorsal= (m) and a =ventral= (l) which pass along the œsophagus (p). The ventral branches of right and left vagi (l and l′) unite a short distance caudad of the root of the lung, and the single trunk extends into the abdominal cavity on the ventral surface of the œsophagus. The dorsal branches of right and left vagi unite farther back, near the diaphragm, and the single trunk thus formed (m) enters the abdominal cavity on the dorsal surface of the œsophagus. Both divisions give fine nerves to the œsophagus.

=Abdominal Portion of the Vagus= (Fig. 164, page 407).--After passing through the diaphragm the ventral division (Fig. 164, l) reaches the lesser curvature of the stomach (1), over which it ramifies, some branches being traceable almost to the pylorus. The network of branches thus formed is the =anterior gastric plexus=. Some twigs from the ventral division pass transversely across the cranial end of the stomach and join the plexus formed by the dorsal division.

The dorsal division (m) of the vagus reaches the greater curvature of the stomach, where it ramifies, forming the =posterior gastric plexus=. Branches from this anastomose with the anterior gastric plexus and with the cœliac plexus of the sympathetic (e).

XI. N. ACCESSORIUS.--The accessory (or spinal accessory) nerve arises (Fig. 138, XI) by numerous rootlets from the lateral surface of the medulla and of the spinal cord as far caudad as the fifth to seventh cervical nerve. These spinal rootlets join to form a nerve which enters the cranium through the foramen magnum. After receiving the rootlets from the medulla, it leaves the cranial cavity along with the vagus and glossopharyngeal by the jugular foramen. Just outside the foramen (Fig. 156, c) it becomes involved in a plexus of fine branches which interconnect it with the vagus, sympathetic, and hypoglossal. It then turns laterad and caudad (Fig. 158, 1) and pierces the cleidomastoid muscle (d), to which it gives small branches. It then divides: one branch enters the sternomastoid muscle; the other passes caudad along the dorsal border of the levator scapulæ ventralis, sends branches to the clavotrapezius, and may be traced to the acromiotrapezius and spinotrapezius, which it supplies.

XII. N. HYPOGLOSSUS (Fig. 156, b).--The twelfth or hypoglossal nerve arises from the ventral side of the medulla (Fig. 138, XII), as previously described. It emerges from the skull through the hypoglossal canal. At first it passes ventrad, then gradually turns craniad, following thus a curved course and passing successively laterad of the vagus and sympathetic nerves (Fig. 156, d and e), the common carotid artery, and the cranial cornu of the hyoid bone. A short distance from its origin the nerve gives off a =ramus descendens= (Fig. 156, f), which receives a communicating branch from the first cervical nerve (j), and divides into two branches, one (g) to the thyrohyoid muscle (9), the other to the sternohyoid (8) and sternothyroid muscles (7). The main nerve passes beneath the mylohyoid muscle into the tongue, at first following the lingual artery, then laterad of it, then crossing it again. It sends branches to all the muscles of the tongue, and may be traced to its tip.