The effect of the bacteria has been determined by experiment to be proportionate to the dose. A cubic centimetre is a cube two-fifths of an inch on each side. One-tenth of such a cube of pure culture of one bacterium (_Proteus vulgaris_) contains 225,000,000 bacteria, and if injected under the skin of a rabbit will produce death. Less than 18,000,000 will produce no effect whatever. Of one kind of staphylococcus, if 250,000,000 are introduced under the skin of a rabbit there will be produced a small abscess, but it requires 1,000,000,000 to produce speedy death. On the other hand, of the bacillus of lockjaw it requires only 1000 to produce death, so virulent is this germ. Moreover, their effect on tissues and persons in different states varies very much. Thus, it is found that when a certain number of bacteria are injected into the cavity of the abdomen of an animal, if the animal is healthy and the peritoneum (the thin lining membrane of the abdomen) is healthy, the animal will recover perfectly well; but if the peritoneum be scraped and torn (and it must be remembered that the healthy peritoneum is devoid of sensation), that the same dose which before was harmless will now produce a violent peritonitis and very likely death. The practical lesson from this experiment upon animals is very evident. Every surgeon who opens the abdomen is most careful, if possible, not to injure the peritoneum, but manipulates with the greatest care lest fatal results follow any serious injury to that membrane. So, too, if the general health be impaired, it is found that an injection from which a healthy animal would recover will be followed by fatal consequences if the general health is below par. Again, if an animal has a simple fracture of his thigh-bone, and that is the only injury that he receives, no infection from the exterior having occurred, he will make a good recovery; but if at the same time he receives a lacerated wound, it may be even in another part of the body, and this wound, not being cared for most scrupulously, becomes infected, the infection will fasten on the distant spot of least resistance, the broken thigh-bone, and will produce a most dangerous and very frequently fatal form of inflammation. I need scarcely point out in this connection, as in fact throughout this entire consideration of bacteriology, how important a part in its development has been played by experiment upon animals. The experimental facts just stated are of vital importance in the treatment of surgical diseases, and evidently could not have been determined upon mankind. It is not too much to say that had vivisection been restricted or prohibited the surgery of to-day would be the barbarous surgery of thirty years ago.