The section is cut at right angles to the long axis of the neck,
passing through the junction of the second and third cervical
vertebræ. _S.c._, Spinal cord; _S.v._, spinal vein; _V.v._, _V.a._,
vertebral vein and artery; _T.p._, transverse process (tip); _F_,
intervertebral fibro-cartilage; _T_, atlantal tendon common to
mastoido-humeralis, splenius, and lower part of trachelo-mastoideus;
_M_, digitation of mastoido-humeralis inserted by _T_.
]
The =bifurcation= of the trachea (Bifurcatio tracheæ) is situated
opposite to the fifth intercostal space, and about five or six inches
(ca. 12 to 15 cm.) below the seventh and eighth thoracic vertebræ.
The trachea is composed of—(1) A fibro-elastic membrane in which are
embedded (2) the cartilaginous rings; (3) a muscular layer; (4) the
mucous membrane. The =elastic membrane= is intimately attached to the
perichondrium of the rings. In the intervals between the latter it
constitutes the annular ligaments (Ligamenta annularia). The =rings= of
the trachea (Cartilagines tracheales) are composed of hyaline cartilage.
They are incomplete dorsally, and when their free ends are drawn apart,
resemble somewhat the letter C. In the cervical part the thin wide free
ends overlap, while in the thoracic part they do not meet; here the
deficiency is made up by thin plates of variable size and form, embedded
in a membrana transversa. Ventrally the rings are about one-half inch
(ca. 1 to 1.2 cm.) wide and much thicker than dorsally. The first ring
is attached to the cricoid cartilage by the crico-tracheal membrane, and
is usually fused dorsally with the second ring. Various irregularities,
such as partial bifurcation or partial or complete fusion with an
adjacent ring, are common. The arrangement in the terminal part is very
irregular. The =muscular layer= (Musculus trachealis) consists of
unstriped fibers which stretch across the dorsal part of the tube. It is
separated from the ends of the rings and the membrana transversa by a
quantity of areolar tissue. When it contracts the caliber of the tube is
diminished. The =mucous membrane= is pale, normally, and presents
numerous fine longitudinal folds, in which are bundles of elastic
fibers. The epithelium is stratified columnar ciliated. Numerous tubular
=mucous glands= (Glandulæ tracheales) are present.
=Blood-supply.=—Common carotid arteries.
=Nerve-supply.=—Vagus and sympathetic nerves.