Part 34

_b._ =N. temporalis profundus.=--One or more large branches passing mediad of the zygomatic arch to the temporal muscle.

_c._ =N. massetericus.=--The masseteric nerve passes dorsocraniad to the masseter muscle.

_d._ =N. pterygoideus.=--One or more pterygoid branches pass to the pterygoid muscles. A small twig from the nerve to the internal pterygoid passes into the tympanic cavity and supplies the tensor tympani muscle.

_e._ =N. buccinatorius= (Fig. 154, _e_).--This passes craniad along the dorsolateral surface of the pterygoid muscles (Fig. 154, 8) to the angle of the mouth. Here it divides into branches to the masseter muscle, the mucosa of the mouth, and to the lips.

After giving off the above branches the mandibular nerve passes three or four millimeters laterad along the root of the zygomatic process of the temporal bone, and divides into two large branches, the =inferior alveolar= (or dental) nerve (_d_) and the =lingual= nerve (_b_).

_f._ =N. alveolaris inferior= (Fig. 154, _d_).--This passes toward the mandibular foramen, but before reaching it gives off a branch to the mylohyoid and digastric muscles. It then enters the foramen, passes through the mandibular canal, and gives numerous =inferior dental= branches to the teeth. At the mental foramen (or foramina) it passes out of the canal and divides into several =mental= nerves, to the chin, lower lip, and mucosa of the lower jaw.

_g._ =N. lingualis= (Fig. 154, _b_).--The lingual nerve passes between the internal and external pterygoid muscles, and two or three millimeters from its origin it receives the =chorda tympani= nerve (_c_), a communicating branch from the facial (see page 376). It passes onto the ventral surface of the external pterygoid, thence onto the side of the tongue, beneath the mylohyoid. On the side of the tongue it divides into branches which enter the tongue and are distributed to its mucous membrane. One branch passes to the mucosa of the pharynx. Just before entering the tongue a branch is given off to the sublingual and submaxillary glands.

VI. N. ABDUCENS.--The sixth nerve, the abducens, arises from the medulla, as already described (Fig. 138, _VI_, and page 347). It passes into the orbit through the orbital fissure, then extends obliquely cranioventrad along the medial surface of the lateral rectus muscle (Fig. 154, _k_). At about the middle of the length of the muscle it divides into two or three branches which enter at once into the lateral rectus and supply it.

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).

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.)

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.

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.

The hypoglossal nerve is involved in the plexus just outside of the jugular foramen, with which the vagus, sympathetic, and accessory nerves are also connected.

2. =Spinal Nerves.=--The origin and general features of the spinal nerves have been described in connection with the account of the spinal cord (page 337). A description of their peripheral distribution will now be given.

A. CERVICAL NERVES.--There are eight pairs of cervical nerves. The first leaves the vertebral canal through the atlantal foramen. The second passes out between the arches of the atlas and axis, not through a special intervertebral foramen, so that its ganglion lies among the muscles of the back of the neck. The others emerge through the intervertebral foramina, the eighth one from between the last cervical and first thoracic vertebræ.

=Dorsal Rami= (Rami posteriores).--The dorsal ramus of the first nerve (N. suboccipitalis) supplies the short dorsal muscles which move the head and connect the atlas and skull. In the second nerve the dorsal ramus is much larger, forming =N. occipitalis major=. It sends small branches to the muscles about its origin, then turns craniad on the surface of the obliquus superior muscle, passes through the biventer cervicis and splenius, joins a small branch from the third nerve, and reaches the dorsal surface of the back of the head. It passes craniad, lying beneath the levator auris longus, emerges from between the two divisions of this muscle, and supplies the skin and cutaneous muscles between the two external ears. The dorsal rami of the other cervical nerves supply muscles and integument on the back of the neck.

=Ventral Rami= (=rami anteriores=).--These pass ventrad between the transverse processes of the vertebræ, except in the case of the =first= nerve (Fig. 156, _j_). This passes ventrad from the atlantal foramen along the groove for the vertebral artery, through the notch in the wing of the atlas, and across the lateral surface of the longus capitis muscle (5). Here it sends a branch caudad to join the second cervical (_k_), then crosses the vagus (_d′_) and sympathetic and the carotid artery, giving off communicating branches to the vagus and sympathetic, and uniting with a branch of the descending ramus of the hypoglossal (_f_). At the lateral surface of the larynx it turns caudad, following the lateral border of the sternothyroid muscle (7), and is distributed to the sternohyoid (8) and sternothyroid (7).

The ventral ramus of the =second= nerve (Fig. 158, 2) receives a branch from that of the first nerve, passes laterad between the levator scapulæ ventralis (_c_) and the cleidomastoid (_d_), receives a communicating branch from the third cervical (3), sends a branch to N. accessorius (1) and numerous small nerves into the sternomastoid (_e_) and cleidomastoid (_d_), then turns craniad and divides into =N. auricularis magnus= (5) and =N. cutaneus colli= (6).

=N. auricularis magnus= (5), the great auricular nerve, passes dorsocraniad across the lateral surface of the sternomastoid (_e_) to the lateral and caudal surface of the external ear and parotid gland (_f_), where it ramifies. The =cutaneus colli= (6) is the smaller, ventral, division of the second nerve; it may receive also an accession from the third. It passes to the integument over the ventral part of M. masseter and ventrad of that muscle.

The =third= nerve (Fig. 158, 3) communicates with the second and supplies the levator scapulæ ventralis (_c_), cleidomastoid (_d_), sternomastoid (_e_), longus capitis, and other muscles of this region and aids in forming the cutaneus colli (6). The =fourth= (4) and =fifth= are distributed to the muscles and integument of the sides of the neck. A branch of the fourth supplies the integument in the hollow of the shoulder, and one from the fifth follows the vena cephalica and supplies the integument over the shoulder. The fifth by sending a branch to aid in forming the phrenic nerve (Fig. 157, _f_) may be considered to enter partly into the brachial plexus.

Owing to the intercommunicating branches between the ventral roots of the first five cervical nerves, these are sometimes considered as forming a loose plexus which receives the name =cervical plexus=.

The sixth, seventh, and eighth cervical nerves (with a part of the fifth) become interconnected with each other and with the first thoracic to form the =brachial plexus=.

=The Brachial Plexus= (Fig. 159).--The brachial plexus is formed by the ventral rami of the fifth, sixth, seventh, and eighth cervical nerves and the first thoracic. Of the fifth cervical only a small part enters into the plexus, forming part of the phrenic nerve. The formation of the plexus is due to the union of the different nerves by means of strong connecting branches or =ansæ=. The plexus lies in the axilla, along with the axillary artery and vein; all its component nerves pass laterad in front of the first rib. Its branches supply the arm and shoulder.