the much more difficult subject of the transmission of disease from
one individual to another. There are four ways by which extension in
the individual may take place.

1. By continuity of tissue, an adjoining tissue or organ becoming
infected by the extension of a focus of infection.

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