ubes, which terminate in the bronchial cells, (_air-cells_,)
lymphatics, and the divisions of the pulmonary artery and veins. All
of these are connected by cellular tissue, which constitutes the
_pa-ren'chy-ma_. Each lung is retained in its place by its _root_,
which is formed by the pulmonary arteries, pulmonary veins, and
bronchial tubes, together with the bronchial vessels and pulmonary
nerves.

468. The TRACHEA extends from the larynx, of which it is a continuation,
to the third dorsal vertebra, where it divides into two parts, called
bronchia. It lies anterior to the spinal column, from which it is
separated by the oesophagus.

469. The BRONCHIA proceed from the bifurcation, or division of the
trachea, to their corresponding lungs. Upon entering the lungs, they
divide into two branches, and each branch divides and subdivides, and
ultimately terminates in small sacs, or cells, of various sizes, from
the twentieth to the hundredth of an inch in diameter. So numerous are
these bronchial or air-cells, that the aggregate extent of their
lining membrane in man has been computed to exceed a surface of 20,000
square inches, and Munro states that it is thirty times the surface of
the human body.

_Illustration._ The trachea may be compared to the trunk of a tree;
the bronchia, to two large branches; the subdivisions of the bronchia,
to the branchlets and twigs; the air-cells, to the buds seen on the
twigs in the spring.

470. The AIR-VESICLES and small bronchial tubes compose the largest
portions of the lungs. These, when once inflated, contain air, under
all circumstances, which renders their specific gravity much less than
water; hence the vulgar term, _lights_, for these organs. The trachea
and bronchial tubes are lined by mucous membrane. The structure of
this membrane is such, that it will bear the presence of pure air
without detriment, but not of other substances.

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467. Of what are the lungs composed? How retained in place? 468. Where
is the trachea situated? 469. Describe the bronchia. What is the
aggregate extent of the lining membrane of the air-cells? To what may
the trachea and its branches be compared? 470. What is said of the
air-cells and bronchial tubes?

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[Illustration: Fig. 91. A representation of the larynx, trachea,
bronchia, and air-cells. 1, 1, 1, An outline of the right lung. 2, 2, 2,
An outline of the left lung. 3, The larynx 4, The tr