surface. The "local lesion" caused by them may be, and often is, trivial in depth and extent, yet the whole organism may be powerfully affected. Moreover, it is not necessarily the tissues in the near neighbourhood of the lesion that are most affected, but frequently tissues far distant. For instance, the microbes of tetanus cause in the neighbourhood of the lesion—e.g. a wound in the hand or foot, to which they have gained entrance—an inflammation of no great extent, and indistinguishable from inflammations due to other causes; but notwithstanding that the local lesion is so slight, the muscles of the body, beginning with those of the head and neck, and passing to those of the trunk and limbs, are thrown into violent contraction, so that the sufferer, resting perhaps on his head and heels only, lies rigid, bent like a bow by the spasm of his strong posterior muscles. The main, the distinctive pathological effect, is therefore not due to any direct action of the micro-organisms, but to a soluble poison (toxin) produced by them, which enters the blood stream at the point of the lesion, and pervades and poisons the whole body, acting probably, like strychnine, chiefly on certain nerve-cells which have for their functions the control of the muscles. Similarly as regards most other zymotic diseases, whether the micro-organisms enter the blood stream or not, the pathological effects produced are directly traceable, not to the micro-organisms, but to their toxins, as is proved by the fact that if these microorganisms are cultivated, as in many instances they have been, in a suitable artificial medium outside the body, and then killed or separated—e.g. by filtration—from the medium, the latter, since it contains the specific toxins, when injected into the body produces all the distinctive symptoms of the disease.
Now, it has been found in the case of various zymotic diseases of which one attack confers immunity,
