The caliber of the trachea is relatively small, its width being about an
inch and a half (ca. 4 cm.) and its height one and a half to two inches
(ca. 4 to 5 cm.). The rings are smaller, and number about fifty. In the
neck their free ends are separated by a varying interval, so that the
tube is flattened and membranous dorsally. In the thorax the ends are in
apposition, so that they form a ridge dorsally. The trachea is adherent
to the right lung from the second rib backward. The bifurcation is
opposite the fourth intercostal space.
[Illustration: