650 THE NERVOUS SYSTEM. 
stinal spaces, and their relations are different on the two sides. (a) In the superior 
mediastinum the right nerve continues its course alongside the innominate artery and 
the trachea, and behind the right innominate vein and superior vena cava, to the 
back of the root of the lung. The /ef?t nerve courses downwards between the left 
common carotid and subclavian arteries, and behind the left innominate vein and 
the phrenic nerve. It passes over the aortic arch, and then proceeds to the back 
of the root of the left lung. (0) Jn the posterior mediastinum the pneumogastric 
nerves are concerned in the formation of two great plexuses—the pulmonary and 
the cesophageal. Behind the root of each lung the nerve breaks up to form the large 
posterior pulmonary plexus, from the lower end of which two nerves emerge on each 
side. These nerves on the right side pass obliquely over the vena azygos major; 
on the left side they cross the thoracic aorta. Both series reach the cesophagus, 
and divide into small anastomosing branches which form the esophageal plexus. At 
the cesophageal opening of the diaphragm the two nerves become separated from 
the plexus, and entering the abdomen—the left nerve in front of the cesophagus, 
the right nerve behind it—they terminate by supplying the stomach and other 
abdominal organs. 
The communications and branches of the pneumogastric nerve may be described 
as (1.) ganglionic, (ii.) cervical, (111.) thoracic, and (iv.) abdominal (Fig. 467). 
The ganglion of the root (g. jugulare) is small and spherical. It occupies 
the jugular foramen, and gives off two branches—meningeal and auricular. 
The meningeal branch passes backwards to supply the dura mater of the posterior 
fossa of the base of the skull. 
The auricular branch (Arnold’s nerve) ascends to the ear in a fissure between the 
jugular and stylo-mastoid foramina. It receives near its origin a twig from the 
tympanic branch of the glosso-pharyngeal nerve, and usually communicates with 
the facial nerve by a pranch arising trom the latter in the aqueduct of Fallopius. 
The nerve is distributed to the back of the pinna and the external auditory meatus, 
and communicates superficially with the posterior auricular nerve. 
Communications.—Besides supplying the meningeal and auricular branches, the 
ganglion of the root of the pneumogastric nerve receives communications from (1) the 
superior cervical ganglion of the sympathetic ; (2) the spinal accessory nerve ; and (3) the 
petrous ganglion of the glosso-pharyngeal nerve (sometimes). 
The ganglion of the trunk of the nerve (g. nodosum), placed immediately 
below the preceding, is large and fusiform. Like the previous ganglion, it supplies 
two branches—the pharyngeal and superior laryngeal nerves. 
The pharyngeal branch receives its fibres (through the ganglion) from the spinal 
accessory nerve. It passes obliquely downwards and inwards to the pharynx 
between the internal and external carotid arteries, and combines with the pharyn- 
geal nerves from the glosso-pharyngeal and superior cervical ganglion of the 
sympathetic to form the pharyngeal plexus. From this plexus the muscles of the 
pharynx and soft palate (except the stylo-pharyngeus and tensor palati) are 
supphed. The lingual branch is a small nerve which separates itself from the 
plexus and joins the hypoglossal nerve in the anterior triangle of the neck. 
The superior laryngeal nerve (n. laryngeus superior) passes obliquely down- 
wards and inwards, behind the external and internal carotid arteries, towards the 
thyroid cartilage. It divides in its course into two unequal parts—a larger internal 
and a smaller external laryngeal nerve. 
The internal laryngeal nerve (ramus internus) passes inwards into the larynx 
between the middle and inferior constrictor muscles of the pharynx and through the 
thyro-hyoid membrane. It supplies the mucous membrane of the larynx, reaching 
upwards to the epiglottis and base of the tongue, and forms communications beneath 
the ala of the thyroid cartilage with the branches of the inferior laryngeal nerve. 
The external laryngeal nerve (ramus externus) passes downwards upon the 
inferior constrictor muscle of the pharynx. It supplies branches to that muscle, 
and ends in the crico-thyroid muscle. 
Communications.—Besides supplying these pharyngeal and laryngeal nerves, the 
ganglion of the trunk of the pneumogastric has the following communications with other 
