cteria
of substances called _opsonins_ which are contained in it, and
the formation of which can be very greatly stimulated. Again, not all
inclusion of bacteria within leucocytes is indicative of phagocytosis;
in many cases the bacteria seem to find the best conditions for
existence within the leucocytes, and these and not the bacteria are
destroyed.

So far it has been shown that the best defence of the body is, as is
the best defence in war, by offensive measures, as illustrated by
phagocytosis and destruction by the serum. Both of these actions can
be increased by their exercise just as the strength of muscular
contraction can be increased by exercise, and the facility for doing
everything increased by habit. Certain of the infectious diseases are,
as has been said, essentially toxic in their nature, and in cultures
the organisms produce poisonous substances. By the injection into the
tissues of such substances the same disturbances are produced as when
the bacteria are injected. Such a disease is diphtheria. In this there
is only a superficial invasion of the tissues. The diphtheria bacilli
are located on the surface of the tonsils or pharynx or windpipe,
where, as a result of their action, the membrane so characteristic of
the disease is produced. The membrane may be the cause of death when
it is so extensively formed as to occlude the air passages, but the
prominent symptoms of the disease, the fever, the weakness of the
heart and the great prostration are due not to the presence of the
membrane, but to the action of toxic substances which are formed by
the bacteria growing in the superficial lesions and absorbed. Tetanus,
or lockjaw, is another example of these essentially toxic diseases.
The body must find some means of counteracting or destroying these
injurious toxic substances. It does this by forming antagonistic
substances called antitoxines, which act not by destroying the
toxines, but by uniting with them, the compound substance being
harmless. It has been found that the production of antitoxine can be
so stimulated by the injection of toxine that the blood of the animal
used for the purpose contains large amounts of antitoxine. The horse
is used in this way to manufacture antitoxine, and the serum injected
into a patient with diphtheria has a curative action, a greater amount
being thus introduced than the patient can manufacture.

[Illustration: FIG. 18.--DIAGRAM TO ILLUSTRATE EHRLICH'S THEORY OF
AN