there was no halt of the infection in the armpit, for all the lymph
nodes there had been removed several years before in the course of an
operation for a tumor of the breast. A general infection of the blood
took place, there was very high fever, and death followed in a few
days. The halt of the infection is important in allowing time for the
body to make ready its means of defence. One cannot avoid comparing
the lymph node with a strong fortress thrown in the path of a
victorious 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.