The =trachea= extends from the larynx to the roots of the lungs, where
it divides into the right and left bronchi. It is kept permanently open
by a series of about fifty to fifty-five incomplete cartilaginous rings
embedded in its wall. It occupies a median position, except near its
termination, where it is pushed a little to the right by the arch of the
aorta. It is approximately cylindrical, but its cervical portion is for
the most part depressed dorso-ventrally by contact with the longus colli
muscle, so that the dorsal surface is flattened. The average caliber is
about two to two and one-half inches (ca. 5 to 6 cm.), but in the
greater part of the neck the transverse diameter is greater and the
dorso-ventral smaller. It is inclosed by a fascia propria.
In its =cervical part= the trachea is related dorsally to the œsophagus
for a short distance, but chiefly to the longus colli muscle. Laterally
it is related to the thyroid gland, the carotid artery, the jugular
vein, the vagus, sympathetic, and recurrent laryngeal nerves, and the
tracheal lymph ducts and cervical lymph glands. The œsophagus lies on
its left face from the third cervical vertebra backward. The
sterno-cephalicus muscles cross it very obliquely, passing from the
ventral surface forward over its sides, and diverging to reach the
angles of the jaw. The omo-hyoidei also cross it very obliquely, passing
over the lateral surfaces of the tube, and converging ventrally to the
body of the hyoid bone. The sterno-thyrohyoideus lies on the ventral
surface.[125] The scaleni lie on either side near the entrance to the
thorax.
The =thoracic part= of the trachea passes backward between the pleural
sacs and divides into the two bronchi over the left atrium of the heart.
It is related dorsally to the longus colli for a short distance, and
beyond this to the œsophagus. Its left face is crossed by the aortic
arch, the left brachial artery, and the thoracic duct. Its right face is
crossed by the vena azygos, the dorso-cervical and vertebral vessels,
and the right vagus nerve. Ventrally it is related to the anterior vena
cava, the brachiocephalic and common carotid trunks, and the left
recurrent nerve.
[Illustration: