The vagus or pneumogastric nerves are the longest of the cranial nerves, and they are remarkable for their almost vertical course, their asymmetry, and their extensive distribution, for, in addition to supplying the lung and stomach, as the name ' pneumo-gastric ' indicates, each nerve gives branches to the external ear, the pharynx, the larynx, the trachea, the oesophagus, the heart, and the abdominal viscera. They are commonly referred to as the tenth pair of cranial nerves.

Each nerve is attached to the side of the medulla, in the postero-lateral sulcus, dorsal to the olivary body, by from twelve to fifteen root filaments which are in linear series with the filaments of the glosso-pharyngeal nerve. The filaments contain both sensory and motor fibers. They pierce the pia mater, from which they receive sheaths, and, traced outward, they pass into the posterior fossa of the cranium toward the jugular foramen and unite to form the trunk of the nerve, which passes through openings in the arachnoid and the dtira mater which are common to it and to the spinal accessory nerve. In the jugular foramen a small spherical ganglion, the jugular ganglion (ganglion of the root) , is interposed in the trunk which here turns at right angles to its former course and descends through the neck. As it leaves the jugular foramen it is joined by the internal or accessory portion of the spinal accessory nerve, and immediately below this junction it enters a large ovoid ganglion, the ganglio7i nodosum or ganglion of the trunk. As it descends through the neck the nerve passes ventral and somewhat lateral to the superior cervical sympathetic ganglion, and in front of the longus capitis and longus colh, from which it is separated by the prevertebral fascia. In the upper part of the neck it is placed between the internal carotid artery and the internal jugular vein, and on a plane dorsal to them, the artery being ventral and mesial, and the vein ventral and lateral. In the lower part of the neck it occupies a similar position in regard to the common carotid artery and the internal jugular vein, and the three structures are enclosed in a common sheath derived from the deep cervical fascia, but within the sheath each structure occupies a separate compartment. In the root of the neck and in the thorax the relations of the nerves of the two sides of the body differ somewhat, and they must, therefore, be considered separately.

The right vagus passes in front of the first part of the right subclavian artery in the root of the neck and then descends in the thorax, passing obliquely downward and backward on the right of the trachea, and behind the right innominate vein and the superior vena cava, to the back of the root of the right lung. Just before it reaches the right bronchus it lies close to the medial side of the vena azygos as the latter hooks forward over the root of the lung. At the back of the right bronchus the right vagus breaks up into a number of branches which join with the branches of tlie sympathetic to form the right posterior pulmonary plexus, and from this plexus it issues in'the form of one or more cords, combined .sensory, visceral motor and sympathetic, which descend on the cesophagus and break up into branches which join with branches of the left vagus, forming the posterior oesophageal plexus. At the lower part of the thorax fibers of this plexus become again associated in one trunk which passes througli the diaphragm on the posterior surface of the oesophagus, and is distributed to the posterior surface of the stomach and to the coeliac (solar) plexus and its offsets.

The left vagus descends through the root of the neck between the carotid and subclavian arteries and in front of the thoracic duct. In the upper part of the superior mediastinum it ia crossed in front by the left phrenic nerve, and in the lower part of the same region it crosses in

Diagram of the Branches of the Vagus Nerves.

front of the root of the subclavian artery and the arch of the aorta and behind the left superior intercostal vein. Below the aortic arch it passes behind the left bronchus and divides into branches which unite with twigs of the sj'mpathetic to form the left posterior pulmonary plexus. Prom this plexus the fibers of the left vagus issue as one or more cords that break up into anastomosing branches to form the anterior oesophageal plexus. At the lower part of the thorax this plexus becomes a single trunk, which passes through the diaphragm on the anterior surface of the oesophagus, and it is distributed to_the anterior surface of the stomach and tothe liver.

The jugular ganglion (ganglion of the root) is a spherical grey mass about five miUimetres in diameter which lies in the jugular foramen. It is connected with the spinal accessory nerve and with the superior cervical sympathetic ganglion, and it gives off an auricular branch, by means of which it becomes associated with the facial and glosso-pharyngeal nerves, and a recurrent meningeal branch.

The ganglion nodosum (ganglion of the trunk) lies below the base of the skull and in front of the upper part of the internal jugular vein. It is of flattened ovoid form and about seventeen miUimetres long and four millimetres broad. It is joined by the accessory part of the spinal accessory nerve, and is associated with the hypoglossal nerve, with the superior cervical ganglion of the sympathetic, and with the loop between the first two cervical nerves, and it gives off a pharyngeal, a superior laryngeal, and a superior cardiac branch. Both ganglia and especially the nodosal retain numerous cell-bodies of sympathetic neurones and the twigs issuing from the ganglia thus contain sympa- thetic fibers. The greater part of the cell-bodies are of sensory neurones.

Communications

The vagus nerve is connected with the glosso-pharyngeal, spinal accessory and hypoglossal nerves, with the sympathetic, and with the loop between the first and second cervical nerves.

(1) Two communications exist between the vagus and glosso-pharyngeal nerves: one between their trunlis, just below the base of the skull, and one, in the region of their gangha, consisting of one or two filaments. When two filaments are present one passes from the jugular gangUon and the other from the auricular nerve to the petrosal ganghon of the glosso-pharyngeal nerve. Either or both of these filaments may be absent.

(2) Two twigs pass from the spinal accessory nerve to the ganglion nodosum, and at a lower level the accessory part of the spinal accessory nerve also joins the same ganglion. The majority of the fibers of the accessory part of the spinal accessory nerve merely pass across the surface of the ganglion and are continued into the pharyngeal and superior laryngeal branches of the vagus, but a certain number blend with the trunk of the vagus and are continued into its recurrent laryngeal and cardiac branches.

(3) Two or three fine filaments connect the ganghon nodosum with the hypoglossal nerve as the latter turns around the lower part of the ganglion.

(4) Fibers pass from the superior cervical ganghon of the sympathetic to both ganglia of the vagus.

(5) A twig sometimes passes from the loop between the first two cervical nerves to the ganglion nodosum.

Terminal branches

These are the meningeal, auricular, pharyngeal, superior laryngeal, recurrent (inferior laryngeal), cardiac, bronchial, pericardial, oesophageal, and the abdominal branches.

(1) The meningeal or recurrent branch is a slender filament which is given off from the jugular ganglion. It takes a recurrent course through the jugular foramen, and is distributed to the dura mater around the transverse (lateral) sinus.

(2) The auricular branch, or nerve of Arnold, arises from the jugular gangUon in the jugular foramen. It receives a branch from the petrosal gangUon of the glosso-pharyngeal, enters the petrous part of the temporal bone through a foramen in the lateral wall of the jugular fossa, and communicates with the facial nerve or merely Ues in contact with it as far as the stylo-mastoid foramen. It usually leaves the temporal bone by the stylo-mastoid foramen, but it may pass through the tympano-mastoid fissure, and it divides, behind the pinna, into two branches, one of which joins the posterior auricular branch of the facial while the other suppUes sensory fibers to the posterior and inferior part of the external auditory meatus and the back of the pinna. It also suppUes twigs to the osseous part of the external auditory meatus and to the lower part of the outer surface of the tympanic membrane.

(3) The pharyngeal branches may be two or three in number. The principal of these joins the pharyngeal branch of the glosso-pharyngeal on the lateral surface of the internal carotid artery, and after passing with the latter medial to the external carotid artery it turns downward and medialward to reach the posterior aspect of the pharynx. Here the two nerves are joined by branches from the superior cervical ganglion of the sympathetic, with which they form the pharyngeal plexus. Branches from this plexus supply sensory fibers to the mucous membrane of the pharynx and motor fibers to the constrictores pharyngis, levator palatini, uvulae, glosso-palatinus, and pharyngo-palatinus.

(4) The superior laryngeal nerve arises from the lower part of the ganghon nodosum, and passes obliquely downward and medialward behind and medial to both internal and external carotid arteries toward the larynx. In this course it describes a curve with the convexity downward and lateralward and divides into (i) a larger internal and (ii) a smaller external branch. Before its division it is joined by twigs with the sympathetic and with the pharyngeal plexus, and it gives a small branch to the internal carotid artery.

(a) The internal branch accompanies the superior laryngeal artery to the interval between the upper border of the thyreoid cartilage and the great cornu of the hyoid bone. It passes under cover of the thyreo-hyoid muscle and pierces the hyo-thyreoid membrane to gain the interior of the pharynx, where it hes in the lateral wall of the sinus piriformis and divides into a number of diverging branches. The ascending branches supply the mucous membrane on both surfaces of the epiglottis, and probably that of a small part of the root of the tongue. The descending branches ramify in the mucous membrane lining the larjmx, and supply the mucous membrane which covers the back of the cricoid cartilage. One of the descending branches passes downward on the internal muscles of the larynx to anastomose with the terminal part of the inferior (recurrent) laryngeal nerve.

(b) The external branch runs downward on the inferior constrictor to the lower border of the th}Teoid cartilage, where it ends, for the most part, in the crico-thyreoid muscle. A few filaments pierce the crico-thyreoid membrane and are distributed to the membrane lining the larynx. It occasionally gives off a cardiac branch which joins one of the cardiac branches of the sympathetic; it also furnishes twigs to the inferior constrictor, and communicating twigs to the pharyngeal plexus, and it receives a communication from the superior cervical gangUon of the sympathetic.

(5) The recurrent (inferior or recurrent laryngeal) nerve of the right side arises from the vagus at the root of the neck in front of the right subclavian artery. It hooks around the artery, passing below and then behind that vessel, and runs upward and slightly medialward, crossing obliquely behind the common carotid artery. Having gained the side of the trachea, it runs upward in the groove between the trachea and the CBsophagus, accompanying branches of the inferior thja-eoid artery, and, near the level of the lower border of the cricoid cartilage, becomes the inferior laryngeal nerve.

In its course the right recurrent nerve gives off branches to the trachea, cesophageal branches to the ojsophagus and pharynx, and, near its commencement, one or more inferior cardiac branches. It communicates with the inferior cervical sympathetic ganghon and with the superior laryngeal nerve.

The inferior laryngeal nerve, the continuation of the recurrent, ascends between the trachea and oesophagus, enters the larynx under cover of the inferior constrictor of the pharynx, and divides into two branches, anterior and posterior. The anterior branch passes upward and forward on the crico-arytajnoideus lateralis and thyreo-arytajnoideus, and supplies these muscles and also the vocalis, arytsenoideus obliquus, ary-epiglotticus, and thyreo-epiglotticus. The posterior branch, passing upward, supplies the crico-arytaenoideus posterior and arytsenoideus obliquus, and anastomoses with the medial branch of the superior laryngeal nerve.

On the left side the recurrent nerve arises in front of the aortic arch and winds around the concavity of the arch lateral to the ligamentum arteriosum. It crosses obliquely behind the root of the left common carotid artery, gains the angular interval between the oesophagus and trachea, and corresponds with the nerve of the right side in the remainder of its course and distribution.

(6) Cardiac branches. - Of these branches of the vagus, there are two sets, the superior and inferior. All the branches of both sets pass to the deep part of the cardiac plexus except a superior branch on the left side that passes to the superficial part of the cardiac plexus. All contain visceral motor, sympathetic and sensory fibers.

(a) The superior (superior and inferior cervical) cardiac nerves arise from the vagus and its branches in the neck. Some of these branches on both sides join with the cardiac branches of the sympathetic in the neck and pass with them to the cardiac plexus. Some on the right side pass independently through the thorax to the deep part of the cardiac plexus, and a branch on the left side passes through the thorax to the superficial part of the cardiac plexus.

(b) The inferior (thoracic) cardiac branches. - These branches on the right side arise in part from the recurrent nerve and in part from the main trunk of the vagus, while on the left side they usually arise entirely from the recurrent. AU these branches pass to the deep part of the cardiac plexus.

(7) The bronchial (pulmonary) branches are anterior and posterior.

(a) The anterior bronchial (pulmonary) branches consist of a few small branches which arise at the upper border of the root of the lung. They pass forward to gain the anterior aspect of the bronchus, where they communicate with the sympathetic and form the anterior pulmonary plexus, from which fine twigs pass along the bronchus.

(b) The posterior bronchial (pulmonary) branches. - Almost the entire remaining trunk of the vagus usually divides into these branches, which join with branches from the second, third, and fourth thoracic ganglia of the sympathetic to form the posterior pulmonary plexus. The plexuses of the two sides join freely behind the bifurcation of the trachea, and branches from the plexus pass along each bronchus into the lung.

(8) The pericardial branches pass from the trunk of the vagus or from the bronchial or oesophageal plexuses to the anterior and posterior surfaces of the pericardium. They are chiefly sensory.

(9) Oesophageal branches, given off by the trunk of the nerve above the bronchial plexuses and from the oesophageal plexuses lower down, pass to the wall of the oesophagus.

(10) Abdominal branches

The terminal part of the left vagus divides into many branches, some of which communicate freely along the lesser curvature of the stomach with filaments from the gastric plexus of the sympathetic, and to some extent with branches of the right vagus, to form the elongated anterior gastric plexus. From this plexus as well as from the nerve-trunk, gastric branches are given to the anterior surface of the stomach. Hepatic branches from the trunk or from this plexus pass in the lesser omentum to the hepatic plexus. The terminal part of the right vagus divides into many branches, and forms along the lesser curvature of the stomach an elongated posterior gastric plexus by communications with branches from the gastric plexus of the sympathetic and with branches from the right vagus. Gastric branches are given off by the trunk of the nerve and from this plexus. Coeliac branches are given by the trunk to the cceliac (solar) plexus, and splenic and renal branches, either directly or through the coeliac (solar) plexus, are given to the splenic and renal plexuses.

Central connections

The sensory fibers of the vagus are processes of the cells of the jugular ganglion and the ganglion nodosum. The peripheral fibers from these cells bring in sensory impulses from the periphery, and their central fibers convey the impulses to the brain. The latter fibers enter the medulla in the filaments of attachment in the postero-lateral sulcus, and, in the reticular formation, they bifurcate into ascending and descending branches which end in the nuclei of termination of the vagus, namely, in the nucleus alae cinerese in the floor of the fourth ventricle and in the nucleus tractus solitarii. The tractus solitarius consists largely of the descending branches. These and the axones arising from the nuclei of termination of the vagus descend the spinal cord to terminate about ventral horn cells which give origin to the phrenic nerve and to motor fibers supplying other muscles of respiration, and they also convey impulses which are distributed to visceral motor neurones along the spinal cord.

The motor fibers spring from the nucleus ambiguus and from the dorsal efferent (motor) nucleus of the vagus join the sensory fibers in the reticular formation. Some of the motor fibers, especially those from the dorsal efferent nucleus, are visceral motor fibers.

The central connections of the vagus are similar to those of the glosso-pharyngeal nerve. Van Gehuchten's observations point to the conclusion that the chief nucleus of termination of the vagus nerve is that of the tractus solitarius.

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