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Page 39
2. By means of lymphatics. Organisms easily enter these vessels which
are in continuity with the tissue spaces and receive the exudate from
the focus of infection. The organisms are carried to the lymph nodes,
which, acting as filters, retain them and for a time prevent a further
extension. The following illustrates the importance of the part the
nodes may play in mechanically holding back a flood of infection. A
physician examined after death the body of a person who died from
infection with a very virulent micrococcus and in the course of the
examination slightly scratched a finger. One of the organs of the body
was removed, sent to a laboratory and received by a laboratory worker,
a woman physician, who had slight abrasions and fissures in the skin
of the hands from contact with irritating chemicals. In the course of
a few hours the wound on the finger of the man became inflamed,
intensely painful, and red lines extended up the arm in the course of
the lymphatic vessels, showing that the organisms were in the
lymphatics and causing inflammation in their course. The lymph nodes
in the armpit into which these vessels empty became greatly inflamed,
swollen, and an abscess formed in them which was opened. There was
high fever, great prostration, a serious illness from which the man
did not recover for several months. The woman only handled the organ
which was sent to the laboratory in order to place it in a fluid for
preservation. She also had a focus of infection of a finger with the
same red lines on the arm, showing extension by the lymphatics; but
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.
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
destruction of the tissue. On the inner surface of these cavities
there may be a rapid growth of bacilli.
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