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
ANTITOXINE FORMATION. The surface of the cell (_n_) is covered with
receptors some of which (_b_) fit the toxine molecule, (_a_) allowing
the toxine to act upon the cell. Under the stimulus of this the cell
produces these receptors in excess which enter into the blood and
there combine with the toxine as in _a^1 b^1_, thus anchoring it and
preventing it from acting upon the cells. The receptors _c_ and _d_ do
not fit the toxine molecule.]

A very ingenious theory which well accords with the facts has been
given by Ehrlich in explanation of the production of antitoxine and of
the reaction between toxine and antitoxine (Fig. 18). This is based on