DISEASES OF THE RESPIRATORY APPARATUS


GENERAL DISCUSSION.--The respiratory apparatus may be divided into two
groups of organs, anterior and posterior. The anterior group, the
_nostrils, nasal cavities, pharynx, larynx_ and _trachea_, is situated in
the region of the head and neck. The posterior group, the _bronchial tubes_
and _lungs_, is situated in the chest or thoracic cavity.

_The nostrils_ are the anterior openings of the air passages. The nasal
cavities are situated in the anterior region of the head, and extend the
entire length of the face. Each cavity is divided into three long, narrow
passages by the two pairs of turbinated bones. The lining membrane is the
nasal mucous membrane, the lower two-thirds or respiratory portion
differing from the upper one-third, in that the latter possesses the nerve
endings of the olfactory nerve and is the seat of smell. The five pairs of
head sinuses communicate with the nasal cavities. Posteriorly and near the
superior extremity of the nasal passages, are two large openings, the
guttural, that open into the pharyngeal cavity.

_The pharynx_ is a somewhat funnel-shaped cavity. The walls are thin and
formed by muscles and mucous membrane. This is the cross-road between the
digestive and respiratory passages. In the posterior portion of the cavity
there are two openings. The inferior opening leads to the larynx and the
superior one to the oesophagus. All feed on its way to the stomach must
pass over the opening into the larynx. It is impossible, however, for the
feed to enter this opening, unless accidentally when the animal coughs. The
cartilage closing this opening is pressed shut by the base of the tongue
when the bolus of feed is passed back and into the oesophageal opening.