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.