they affect many parts and cause the
formation of an enormous number of tubercles. By the union of such
tubercles, masses of tubercular material are formed, which in some cases
are of great size. The disease is called tuberculosis because it is
characterized by the formation of these peculiar nodules, and the bacillus
which causes the disease is for the same reason known technically as the
_Mycobacterium tuberculosis._

There are undoubtedly predisposing conditions which contribute toward the
development of the disease; some of these are found in the animal body and
others in the environment. An enfeebled condition caused by insufficient
feed, exposure to great extremes of atmospheric temperature and insanitary
surroundings, or the drain occasioned by heavy production of milk, appear
to aid the development of the bacillus, and there is also a special
individual susceptibility in some cases which may be otherwise described as
an inability of the animal tissues to resist and destroy the bacilli when
they have penetrated to the inner recesses of the body.

Among the conditions of environment which aid the development of
tuberculosis may be mentioned stabling with lack of ventilation, damp
buildings, the keeping of many animals together, drafts of air which cause
colds and catarrhs, and, in general, everything which prevents the animals
from developing and maintaining the highest condition of health. None of
these conditions of body or environment are sufficient to cause the
disease, however, unless the animals are exposed to the _Mycobacterium
tuberculosis_ and it penetrates the tissues of their bodies.

The ways in which the tubercle bacilli find their way into the body may be
considered under four heads: (1) By inhalation into the lungs; (2) by
taking into the digestive tract in the milk of tuberculous cows or with
other contaminated feed; (3) during coition when the sexual organs are
tuberculous; (4) from the tuberculous mother to the fetus in the uterus.
The bacilli can reach the lungs by inhalation only when the bacilli are
thoroughly dried and pulverized and in condition to be carried by currents
of air.

It is well known that the bacilli withstand drying for months before they
lose their power of producing disease. They leave the bodies of diseased
animals in several ways. There may be a little discharge occasionally
coughed up as a spray from the diseased lungs, or this material may be
swallowed and the bacilli