ases no further extension
of the process takes place. In certain cases, however, the growth of
the bacilli in the focus is unchecked, the tissue about them is killed
and becomes converted into a soft semi-fluid material; further
extension then takes place. All parts of the enormous surface of the
lungs are connected by means of the system of air tubes or bronchi,
and the bacilli have favorable opportunity for distribution, which is
facilitated by sudden movements of the air currents in the lung
produced by coughing. The defence of the body can still keep pace with
the attack, and even in an advanced stage the infection can be checked
in some cases permanently; in others the check is but temporary, the
process of softening continues, and large cavities are produced by the
destruction of the tissue. On the inner surface of these cavities
there may be a rapid growth of bacilli.

From the lungs the bacilli are carried by the lymphatics to the lymph
nodes at the root of the lungs, in which a similar process takes
place; this, on the whole, is favorable, because further extension by
this route is for a time blocked. The extension by means of surfaces
continues, the abundant sputum which is formed in the lungs and which
contains large numbers of bacilli, becomes the vehicle of
transportation. The windpipe and larynx may become infected, the back
parts of each are more closely in contact with the sputum and are the
parts most generally infected. A large part of the sputum is swallowed
and infection of the intestine takes place, the lesions taking the
form of large ulcers. From the intestinal ulcers there is further
extension by means of the lymphatics, to the large lymph nodes in the
back of the abdominal cavity (Fig. 8-25); the bacilli may also pass
from the ulcers into the abdominal cavity and be distributed over the
surface of the peritoneum resulting in tuberculous peritonitis. When
the disease has reached an advanced stage, bacilli in small numbers
continually pass into the blood and are distributed by this over the
body, producing small nodules in many places. In rare cases
distribution by the blood is the principal method of extension, and
immense numbers of small foci of disease are produced, the form of
disease being known as acute miliary tuberculosis. Although the
bacilli are distributed everywhere, certain organs, as the brain and
muscles, are usually exempt, because in these the conditions are not
favorable to fur