The primary seat of the infection in man is generally in the upper
part of the lung. The organisms settle on the surface here and cause
multiplication of the cells and an inflammatory exudate in a small
area. With the continuous growth of the bacilli in the focus,
adjoining areas of the lung become affected, and there is further
extension in the immediate vicinity by means of the lymphatics. Small
nodules are formed and larger areas by their coalescence. Infection
with tuberculosis is so common that at least three-fourths of all
individuals over forty show evidences of it. The examination of two
hundred and twenty-five children of the average age of five years who
had died of diphtheria showed tuberculous infection in one-fifth of
the cases and the frequency of infection increases with age. The
defence on the part of the body is chiefly by the formation of dense
masses of cicatricial tissue which walls off the affected area and in
which the bacilli do not find favorable conditions for growth. This
mode of defence, which is probably combined with the production of
substances antagonistic to the toxines produced by the bacilli, is so
efficacious that in the great majority of cases 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