It permits the free passage of air to and from the lungs.

The _Bronchia_ are two tubes, or branches, one proceeding from the
windpipe to each lung. Upon entering the lungs, they divide and
subdivide until, finally, they terminate in small cells, called the
_bronchial or air-cells,_ which are of a membranous character.

[Illustration: Fig. 43.
An ideal representation of the respiratory organs. _3._ The
larynx. _4._ The trachea. _5, 6._ The bronchia. _9, 9, 9, 9._ Air-cells.
_1, 1, 1, 2, 2, 2._ Outlines of the lungs.]

The _Lungs_ are irregular conical organs rounded at the apex, situated
within the chest, and filling the greater part of it, since the heart is
the only other organ which occupies much space in the thoracic cavity.
The lungs are convex externally, and conform to the cavity of the chest,
while the internal surface is concave for the accommodation of the
heart. The size of the lungs depends upon the capacity of the chest.
Their color varies, being of a pinkish hue in childhood but of a gray,
mottled appearance in the adult. They are termed the _right_ and _left_
lung. Each lung resembles a cone with its base resting upon the
diaphragm, and its apex behind the collar-bone. The right lung is larger
though shorter, than the left, not extending so low, and has three
_lobes_, formed by deep fissures, or longitudinal divisions, while the
left has but two lobes. Each lobe is also made up of numerous _lobules_,
or small lobes, connected by cellular tissue, and these contain great
numbers of cells. The lungs are abundantly supplied with blood-vessels,
lymphatics, and nerves. The density of a lung depends upon the amount of
air which it contains. Thus, experiment has shown that in a _foetus_
which has never breathed, the lungs are compact and will sink in water;