elastic and somewhat rose-colored. Each lung is divided into lobes, and
each lobe into a great number of lobules by the supporting connective
tissue. The lobule is the smallest division of the lung and is formed by
capillary bronchial tubes, air cells and blood-vessels. It is here that the
external respiration or the exchange of gases between the capillaries and
the air cells occurs.

VENTILATION.--It is agreed by all persons who have investigated the
subject, that unventilated stable air is injurious to animals. At one time
it was believed that the injurious effects resulting from the breathing of
air charged with gases and moisture from the expired air and the animal's
surroundings, were due to a deficiency in oxygen. It is now believed that
the ill-effects are mainly due to the stagnation of air, the humid
atmosphere, and the irritating gases emanating from the body excretions.

The common impurities found in _stable air_ are carbonic and ammonia gas,
moisture charged with injurious matter and dust from the floor and bodies
of the animals. As a rule, the more crowded and filthy the stable, the more
impurities there are in the air. If any of the animals are affected with an
infectious disease, such as tuberculosis or glanders, the moisture and dust
may act as carriers of the disease-producing germs. Infectious diseases
spread rapidly in crowded, poorly ventilated stables. The two factors
responsible for this rapid spread of disease are the lowered vitality of
the animal, due to breathing the vitiated air, and the greater opportunity
for infection, because of the comparatively large number of bacteria
present in the air.

_The purpose of stable ventilation_ is to replace the stable air with purer
air. The frequency with which the air in the stable should be changed
depends on the cubic feet of air space provided for each animal, and the