y Leonard Pearson, B. S., V. M. D.]

The organs pertaining to the respiratory function may be enumerated in
natural order as follows: The nasal openings, or nostrils; the nasal
chambers, through which the air passes in the head; the sinuses in the
head, communicating with the nasal chambers; the pharynx, common to the
functions of breathing and swallowing; the larynx, at the top of the
windpipe; the trachea, or windpipe; the bronchi (into which the windpipe
divides), two tubes leading from the windpipe to the right and left
lungs, respectively; the bronchial tubes, which penetrate and convey air
to all parts of the lungs; the lungs.

The pleura is a thin membrane that envelops the lung and lines the walls
of the thoracic cavity. The diaphragm is a muscular structure,
completely separating the contents of the thoracic cavity from those of
the abdominal cavity. It is essentially a muscle of inspiration, and the
principal one. Other muscles aid in the mechanism of respiration, but
the diseases or injuries of them have nothing to do with the diseases
under consideration.

Just within the nasal openings the skin becomes gradually but
perceptibly finer, until it is succeeded by the mucous membrane. Near
the junction of the skin and membrane is a small hole, presenting the
appearance of having been made with a punch; this is the opening of the
lachrymal duct, a canal that conveys the tears from the eyes. Within and
above the nasal openings are the cavities, or fissures, called the false
nostrils. The nasal chambers are completely separated, the right from
the left, by a cartilaginous partition, the nasal septum. Each nasal
chamber is divided into three continuous compartments by two thin,
scroll-like turbinated bones.

The mucous membrane lining the nasal chambers, and, in fact, the entire
respiratory tract, is much more delicate and more frequently diseased
that the mucous membrane of any other part of the body. The sinuses of
the head are compartments which communicate with the nasal chambers and
are lined with a continuation of the same membrane that lines the nasal
chambers; their presence increases the volume and modifies the form of
the head without increasing its weight.

The horse, in a normal condition, breathes exclusively through the
nostrils. The organs of respiration are quite liable to become
diseased, and, as many of the causes which lead to these attacks can be
avoided, it is both important and profitabl