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THE NERVOUS SYSTEM. : 315

2. With the hypoglossal nerve, by two or three filaments.
'>. Generally with the arcade formed by the anterior branches of the
first and second cervical nerves.
4. With the sympathetic system through branches from the superior
cervical ganglion. (Fibres from this ganglion extend to join the main
trunk as it passes downward.)
The pneumogastric nerve passes from its inferior ganglion vertically
down the neck upon the outer wall of the pharynx, behind the internal
jugular vein and internal carotid artery above and the common carotid
below, the nerve, vein, and artery being enclosed in a common sheath.
On passing from the neck into the thorax the right and left nerves do
not folloM' a similar course or bear the same relations with the tissue
with which they come in contact.
The Bight Pneuiiiogadric Nerve enters the thorax between the subcla-
vian vein and the first portion of the subclavian artery. It then passes
between the right innominate vein and the innominate artery, then
behind the arch of the aorta in a groove between the trachea and the
oesophagus to the root of the lung, where it becomes somewhat flattened
and gives oif numerous branches, which are joined by similar branches
from its fellow of the opposite side. These branches together form the
right posterior pulmonary plexus. The nerve is then continued down-
ward by two cords to the posterior surface of the cesophagus, where it
subdivides and communicates with similar subdivisions from the corre-
sponding nerve of the left side forming the oesophageal plexus. From
this plexus the nerve, after receiving fibres from the left pneumogastric,
is formed into a single cord, and passes down the neck in close api)osi-
tion with the posterior surface of the cesophagus, through the dia])liragm,
and reaches the posterior surface of the stomach, "where it spreads out
and distributes branches to the liver and the solar plexus.
The Left Pneumogastric Nerve enters the thorax between the left
common carotid and subclavian arteries, crosses the inner surface of
the innominate vein, and passes down in front of the descending portion
of the arch of the aorta. It will thus be seen that it holds a more ante-
rior position in the thorax than the nerve of the right side. From the
arch of the aorta it passes behind the root of the left lung, spreads out
to receive branches from the right nerve, and forms the left posterior
pulmonary plexus. From this plexus it descends in close apposition to
the anterior surface of the oesophagus, giving oif branches to form the
oesophageal plexus. It then passes as a single trunk in front of the
oesophagus, through the diaphragm to the anterior surfiice of the stomach,
to M'hich it is distributed. It sends branches to the spleen, pancreas,
liver, and small intestines.
The following tabulated arrangement of the branches of the pnemo-
gastric nerve is taken from Prof. Allen's Avato7ni/

-if Auricular.
T^
Encranial,
{Meningeal.
r [Willi the spinal accessory nerve.
T^ • 1 . i i- With the glosso-iiharvngeal nerve.
Excranial, Anastomotic, I
j with the hypoglossal nerve.
L With the sympathetic nerve.
[