us invading army behind which the defenders may gather and
which affords them time to renovate their strength.

3. By means of the blood. The blood vessels are universally
distributed, the smaller vessels have thin walls easily ruptured and
easily penetrated. It is probable that in every infection some
organisms enter the blood which, under usual conditions, is peculiarly
hostile to bacteria. These may, however, be carried by the blood to
other organs and start foci of infection in these.

4. By means of continuous surfaces. The bacteria may either grow along
such surfaces forming a continuous or more or less broken layer, or
may be carried from place to place in the fluids which bathe them.

All these modes of extension are well shown in tuberculosis. This
disease is caused by a small bacillus which does not produce spores,
has no power of saphrophytic growth under natural conditions, and is
easily destroyed. Moisture and darkness are favorable conditions for
its existence, sunlight and dryness the reverse. There are three
varieties or strains of the tubercle bacilli which infect respectively
man, cattle and birds, and each class of animals shows considerable
resistance to the varieties of the bacillus which are most infectious
for the others.

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 c