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THE NEBVOUS SYSTEM. 319

mucous membrane below the rima glottidis. In their passage upward
these nerves distribute small branches to the structure and mucous mem-
brane of the trachea and oesophagus and to the inferior constrictor mus-
cle of the ]>harvnx. As the nerves pass beneath the large arteries of
the neck they send communicating branches to the inferior cervical sym-
pathetic ganglion and to the cardiac plexus, which is formed by the
interlacing of branches from the pneumogastric nerve and sympathetic
system. The right inferior laryngeal nerve occasionally sends a fila-
ment to the pericardium.
llie Cardiac Branche.^ arise as two sets, and receive the name of cer-
vical and thoracic branches.
The Cervical Cardiac Branches are two or three in number (usually
three), two of which arise from the main trunk of the pneumogastric
in the upper region of the neck, and unite with the cardiac branches of
the svmpathetic system as they descend. The third branch arises from
the pneumogastric nerve just before it enters the thorax. On the right
side the nerve passes in front of the brachio-cephalic artery, and unites
with the superior cardiac nerve in its passage to the deep cardiac plexus,
a few filaments passing to the coats of the aorta. On the left side the
nerve passes in front of the arch of the aorta, and unites with the supe-
rior cardiac nerve or passes directly to the superficial cardiac plexus.
TJie Thoracic Cardiac Branches of the right side arise partially from
the trunk of the pneumogastric nerve below the origin of the right
recurrent laryngeal as the nerve lies close to the trachea, and partially
from the recurrent branch of the pneumogastric. They terminate in
the deep cardiac plexus. The branches of the left side usually arise
from the recurrent or inferior laryngeal nerve and terminate in the
superficial cardiac plexus.
The Pulmonary Brandies are separated into two sets, anterior and
posterior.
The Anterior Pulmonary Branches are the smaller of the two sets,
and consist of two or three slender filaments which arise from the pneu-
mogastric nerve below its cardiac branches. A few of these filaments
pass to the trachea before they form, together with the sympathetic
system of the pulmonary artery, the anterior pulmonary plexus.
Fibres from this plexus encircle and pass along the bronchial tubes
to their terminations in the air-cells of the lungs.
The Posterior Pulmonary Branches are larger and more numerous
than the anterior. They arise from the flattened portion of the pneu-
mogastric nerve behind the root of the lung. They unite with filaments
from the second, third, and fourth thoracic sympathetic ganglia to form
the posterior pulmonary plexus. From this plexus a few filaments are
distributed to the inferior and posterior portion of the trachea, to the
muscular tissue and mucous membrane of the central region of the
oesophagus, and a few to the posterior superior portion of the pericar-
dium. The principal portions of these branches, however, surround the
bronchial tubes, and pass along them to the air-cells of the lungs in the
same manner as the branches from the anterior pulmonary plexus. The
anterior and posterior })ulmonary plexuses of each side give off a large
number of communicating branches which pass between each other, so