eir sacs,
air tubes, and blood vessels; the membranes that line their inner and
outer surfaces; and the connective tissue that binds these parts together.
All these tissues are more or less elastic. The relation of the different
parts of the lungs to each other and to the outside atmosphere will be
seen through a study of the

*Air Passages.*--The air passages consist of a system of tubes which form a
continuous passageway between the outside atmosphere and the different
divisions of the lungs. The air passes through them as it enters and
leaves the lungs, a fact which accounts for the name.

[Fig. 34]

Fig. 34--*Model of section through the head*, showing upper air passages
and other parts. 1. Left nostril. 2. Pharynx. 3. Tongue and cavity of
mouth. 4. Larynx. 5. Trachea. 6. Esophagus.

The incoming air first enters the _nostrils_. These consist of two narrow
passages lying side by side in the nose, and connecting with the pharynx
behind. The lining of the nostrils, called _mucous membrane_ is quite
thick, and has its surface much extended by reason of being spread over
some thin, scroll-shaped bones that project into the passage. This
membrane is well supplied with blood vessels and secretes a considerable
quantity of liquid. Because of the nature and arrangement of the membrane,
the nostrils are able to _warm_ and _moisten_ the incoming air, and to
_free it from dust particles_, preparing it, in this way, for entrance
into the lungs (Fig. 34).

The nostrils are separated from the mouth by a thin layer of bone, and
back of both the mouth and the nostrils is the pharynx. The _pharynx_ and
the _mouth_ serve as parts of the food canal, as well as air passages, and
are described in connection with the organs of digestion (Chapter X). Air
entering the pharynx, either by the nostrils or by the mouth, passes
through it into the _larynx_. The larynx, being the special organ for the
production of the voice, is described later (Chapter XXI). The entrance
into the larynx is guarded by a movable lid of cartilage, called the
_epiglottis_, which prevents food particles and liquids, on being
swallowed, from passing into the lower air tubes. The relations of the
nostrils, mouth, pharynx, and larynx are shown in Fig. 34.

From the larynx the air enters the _trachea_, or windpipe. This is a
straight and nearly round tube, slightly less than an inch in diameter and
about four a