First of all, there is _antisepsis_. For centuries and centuries surgeons operated without understanding why it was that death struck down so unmercifully those operated upon. In vain did surgeons display great skill; in vain did the operation succeed: the patient died. Erysipelas, lock-jaw, abscess-formation and gangrene reigned supreme. Every confinement exposed the mother to death; the slightest wounds were followed by the most serious after-effects; in certain amputations, for instance, the mortality was 70 per cent. No one dared to touch either the peritoneum or the joints, because every operation on the peritoneum or on the articulations was sure to prove fatal. But Lister and Pasteur came! These two men, simultaneously and concurrently, demonstrated that all disease following on an operation was the result of infection by parasites. By preventing the wounds from being contaminated by parasites, infection was prevented; for the wounds themselves are innocent, as long as they are not infected. This is the astounding and simple truth which Lister and Pasteur established. And let no one pretend it is so simple that the data could have been furnished by clinical observation alone, for such an assertion would be contradicted by the facts. Thousands and thousands of surgeons, right up to 1868, had understood nothing of infection. In order to understand this big word "infection," which sums up in itself the whole of surgery and the whole of medicine, it was necessary to inject pus into animals, gather the microbes which then developed in the blood of these animals, isolate the microbes, cultivate them, inject them afresh, and produce an experimental disease. It was in this manner only that we were able to understand the mechanism of antisepsis, and, consequently, apply it to the treatment of operations and wounds. Three or four volumes could be written on this subject alone, but all I can attempt here is a summary of the main points. I say without hesitation that as long as clinical medicine confined itself only to the observation of patients, it was able to understand nothing, to analyse nothing, to foresee nothing. It was necessary to experiment, to sacrifice a few hundred mice, rats, and rabbits, in order to demonstrate that erysipelas is an inoculable disease, that puerperal infection is of the same nature as purulent infection, that all these diseases are due to micro-organisms, and that certain substances, called antiseptics, can stop the development of these fatal germs. It appears quite natural to-day (and it seems to simple minds, ignorant of the past and powerless to imagine the past, that these notions have been current from all eternity) to know that instruments, water, and linen heated to 120° contain no living germs. But this discovery is not so very old. It was Pasteur who, between 1863 and 1873, established it by some memorable experiments at the cost of a little disease given to rats and guinea-pigs.