Three generations of Alexander Monros were professors of anatomy at the University of Edinburgh in unbroken succession from 1720 to 1846. All three contributed immeasurably toward making the medical school at Edinburgh among the world's best of its time. The subject of this article is called Monro secundus to distinguish him from his father, Alexander Monro primus (1697-1767), and his son, Alexander Monro tertius (1773-1859). Monro secundus, the greatest of the three, is chiefly remembered for discovering and accurately describing the foramen interventriculare, or "foramen of Monro," an important passage connecting the lateral and third ventricles of the brain. Knowledge of the communicative function of the foramen of Monro became crucial for the new science of neuroradiology early in the twentieth century because, for neurosurgeons like Walter Dandy (1886-1946), it paved the way for useful neurological diagnostic techniques such as pneumoventriculography, which injects air as an x-ray contrast medium directly...