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Practical Medicine is advanced independently of Physiology. There is another way, which I have already mentioned—the immediate application to practice of a single observed fact. The instances are innumerable. They include most of the minor improvements in medicine, and they include improvements of the very highest importance. Such was the observation that the inhalation of ethereal vapours causes a transient insensibility to pain. Such was the observation that lemon-juice prevents scurvy : who can calculate how much suffering, and how many lives, it has saved; how much the commerce of the world and the influence of England owe to this one observation? Such, or nearly such, was the invention of vaccination by Jenner. We cannot rank this among great intellectual achievements; but if we estimate it by its immediate fruits, we must call it, as Dr Baillie did, "the most important discovery ever made in medicine."
He spoke as an eye-witness of small-pox in the days when it raged unchecked in all its loathsomeness and fatality. We at least know for certain, that the lives saved every year by Vaccination, must be reckoned by tens of thousands.
