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MU. YOU ATT S VETERINARY LECTURES. 
At the bottom of the neck the two nerves separate ; and the 
cerebro-visceral climbs above the carotid artery, and is found 
between it and the axillary, and, accompanied by them, enters 
the cavity of the chest between the two first ribs. 
The Cerebro-visceral in the Thorax .—The arrangement of the 
great viscera within the thorax, the inclination of the heart to 
the left side, and the great bulk of the right lung, give a different 
direction to the nerve on the opposite sides of the chest. Both 
are found within the superior mediastinum, but the right nerve 
continues to follow the trachea; it passes over the root of the 
right lung, and reaches the under surface of the oesophagus. 
The left nerve accompanies the anterior aorta, and, crossing the 
root of the posterior aorta, also reaches the oesophagus ; and both 
of them enter the abdomen attached to the oesophagus. The 
divisions of this nerve within the thorax deserve close attention. 
The Tracheal Plexus. —First there are many branches, united 
with a still greater number from the great organic, entwining 
round the lower part of the trachea. The transverse muscle has 
not yet quite ceased to exist; nor the interposed ligamentous 
muscular band connecting together the rings of the trachea: 
while there is a new and complicated mechanism of carti¬ 
lage at the commencement of the bronchial tubes, and also con¬ 
siderable motion or change of situation in this part of the respira¬ 
tory canal during the inflation and subsidence of the lungs. 
We may consider this plexus, so far at least as this nerve is con¬ 
cerned, as presiding over and regulating all these motions and 
changes. Some branches are next given off to the heart, but 
more of these hereafter. 
The Recurrent Nerve. — Somewhat differently originating 
on the opposite sides of the chest, a branch is next given off 
from the cerebro-visceral, which takes a retrograde course ; it 
escapes from the chest, and climbs up the neck as far as the larynx. 
From this singularity of direction it is called the recurrent nerve. 
It gives, at its commencement, fibres both to the pulmonary and 
carotid plexuses, and to the posterior cervical ganglion of the great 
organic; and then, as it pursues its singular course up the neck, 
it sends many filaments to the oesophagus and to the trachea. 
The first seems to be much indebted to it for the power of its mus¬ 
cular coat; and the latter owes to it much of the action of the 
transverse muscle, whether employed in tightening and smoothing 
the lining membrane of the trachea in order to facilitate the pas¬ 
sage of the air, or in preserving the arch of the windpipe from 
alteration of form, or from laceration. The power of the liga- 
mento-muscular substance interposed between the rings of the 
trachea must also be traced to the same source. 
