3. The Trachea.— The trachea is about four and a
half inches in length and three-quarters of an inch wide,
and extends from the lower border of the cricoid car-
tilage, on a level with the upper border of the sixth cervical
vertebra, to the lower border of the fourth, or, to the mid-
dle of the fifth dorsal vertebra. The principal relations
in front of the trachea are the arch of the aorta and its
branches, the latter lying directly in front of the trachea
at their origins, but diverging above, so as to lie partially
on either side, at their terminations, and the former, 7.e.,
the arch itself passing obliquely across the bifurcation,
the deep cardiac plexus lying between. Behind, the tra-
chea rests on the cesophagus and divides into its two
bronchi, right and left; the former, one inch in length, is
nearly horizontal in direction, and enters the lung, about
the fifth dorsal vertebra, while the latter, two inches long,
passes downwards and to the left and enters the lung at
about the level of the sixth vertebra, after running under
the left portion of the arch of the aorta.
4. The Oesophagus, about nine inches long, begins
at the upper border of the cricoid cartilage, and about on
a level with the intervertebral disk between the fifth and
sixth cervical vertebrz. It passes down the neck deviat-
ing to the left, and, on entering the superior mediasti-
num, inclines towards the middle line again, which it


152 Anatomy Applied to Medicine and Surgery.
