nd one half inches in length. Its walls contain from sixteen
to twenty C-shaped, cartilaginous rings, one above the other and
encircling the tube. These incomplete rings, with their openings directed
backward, are held in place by thin layers of connective and muscular
tissue. At the lower end the trachea divides into two branches, called the
bronchi, each of which closely resembles it in structure. Each _bronchus_
separates into a number of smaller divisions, called the _bronchial
tubes_, and these in turn divide into still smaller branches, known as the
_lesser bronchial tubes_ (Fig. 33). The lesser bronchial tubes, and the
branches into which they separate, are the smallest of the air tubes. One
of these joins, or expands into, each of the minute lung sacs, or
infundibula. Mucous membrane lines all of the air passages.

*General Condition of the Air Passages.*--One necessary condition for the
movement of the air into and from the lungs is an unobstructed
passageway.(31) The air passages must be kept open and free from
obstructions. They are _kept open_ by special contrivances found in their
walls, which, by supplying a degree of stiffness, cause the tubes to keep
their form. In the trachea, bronchi, and larger bronchial tubes, the
stiffness is supplied by rings of cartilage, while in the smaller tubes
this is replaced by connective and muscular tissue. The walls of the
larynx contain strips and plates of cartilage; while the nostrils and the
pharynx are kept open by their bony surroundings.

[Fig. 35]

Fig. 35--*Ciliated epithelial cells.* _A._ Two cells highly magnified. _c._
Cilia, _n._ Nucleus. _B._ Diagram of a small air tube showing the lining
of cilia.

The air passages are _kept clean_ by cells especially adapted to this
purpose, known as the _ciliated epithelial cells_. These are slender,
wedge-shaped cells which have projecting from a free end many small,
hair-like bodies, called _cilia_ (Fig. 35). They line the mucous membrane
in most of the air passages, and are so placed that the cilia project into
the tubes. Here they keep up an inward and outward wave-like movement,
which is quicker and has greater force in the _outward_ direction. By this
means the cilia are able to move small pieces of foreign matter, such as
dust particles and bits of partly dried mucus, called phlegm, to places
where they can be easily expelled from the lungs.(32