Chapter 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_).