Any person holding a diploma from a reputable school of medicine and engaged in the active practice of his profession, is in law an expert. In this capacity he may be summoned at any moment to testify to questions of fact, hypothetical or theoretical. The questions thus propounded to the medical witness are frequently complex in their nature, involve a wide range of inquiry, and necessitate on his part a just discrimination, extensive knowledge and large experience. Again, medical science is ever varying; it may be likened to an uncertain stream that shifts its banks--restless and aggressive, the land-marks change, but the river's course is ever onward. Principles like the rocks left in its ancient bed, alone remain to mark its passage and reveal its work; accepted truths of to-day may be _un_truths to-morrow. Errors have been enunciated by Philosophers, have been sanctified by the Church, and promulgated by Priests, but have finally been overtaken by this same resistless stream of progress, and by it have been swept out of the world. Even so to-day our science is changing its foundation stones. Insanity is but just emerging from a complex labyrinth of metaphysical obscurities, and has taken its place in pathology as a physical disease. Physiological Chemistry has scarcely conned its alphabet, and its unknown literature, pregnant with marvelous truths has yet to unfold its treasures to us. Equally unexplored is the vast field embraced in the aetiology of diseases, the character of morbid germs and their mode of entry into the economy. Organic Chemistry is filling our libraries with its new facts and experiments. The imperative demand therefore of the medical expert is constant study. The exigencies of the position require, in justice to the profession, a thorough acquaintance with all that is old, and an equal familiarity with all that is new. The range of judicial inquiry often embraces the entire field of medical and surgical knowledge, as well as all their collateral branches. No obsolete theory, no unborn or possible fact is too remote for searching investigation. Hypothetical questions, ingeniously framed, which include complex and unusual possibilities or specious probabilities, invite the attention, and tax the knowledge, the memory and the judgment of the expert.