Then one day, a lassoed mad dog was brought into the laboratory; dangerously he was slid into a big cage with healthy dogs and allowed to bite them. Roux and Chamberland fished froth out of the mouth of this mad beast and sucked it up into syringes and injected this stuff into rabbits and guinea-pigs. Then they waited eagerly to see this menagerie develop the first signs of madness. Sometimes--alas--the experiment worked, but other very irritating times it did not; four healthy dogs had been bitten and six weeks later they came in one morning to find two of these creatures lashing about their cages, howling--but for months after that the other two showed no sign of rabies; there was no rime or reason to this business, no regularity, confound it! this was not _science_! And it was the same with the guinea-pigs and rabbits: two of the rabbits might drag out their hind legs with a paralysis--then die in dreadful convulsions, but the other four would go on chewing their greens as if there were no mad-dog virus within a million miles of them. Then one day a little idea came to Pasteur, and he hurried to tell it to Roux. “This rabies virus that gets into people by bites, it settles in their brains and spinal cords.... All the symptoms of hydrophobia show that it’s the nervous system that this virus--this bug we can’t find--attacks.... “That’s where we have to look for the unknown microbe ... that’s where we can grow it maybe, even without seeing it ... maybe we could use the living animal’s brain instead of a bottle of soup ... a funny culture-bottle that would be, but.... “When we inject it under the skin--the virus may get lost in the body before it can travel to the brain--if I could only stick it right into a dog’s brain...!” Roux listened to these dreamings of Pasteur, he listened bright-eyed to these fantastic imaginings.... Another man than Roux might have thought Pasteur completely crazy.... The brain of a dog or rabbit instead of a bottle of broth, indeed! What nonsense! But not to Roux! “But why not put the virus right into a dog’s brain, master, I can trephine a dog--I can drill a little hole in his skull--without hurting him--without damaging his brain at all ... it would be easy ...” said Roux. Pasteur shut Roux up, furiously. He was no doctor, and he did not know that surgeons can do this operation on human beings even, quite safely. “What! bore a hole right through a dog’s skull--why, you’d hurt the poor beast terribly ... you would damage his brain ... you would paralyze him.... No! I will not permit it!” So near was Pasteur, by reason of his tender-heartedness, so close was he to failing completely in winning to the most marvelous of his gifts to men. He quailed before the stern experiment that his weird idea demanded. But Roux--the faithful, the now almost forgotten Roux--saved him by disobeying him.