Recognizing pathology as simply morbid physiology--that is, the study of the body and its functions in states of disorder from morbid conditions--how these morbid conditions are produced is the complex question to be answered by Etiology. Nor is this question (or series of questions) by any means only of speculative or theoretical importance. It is, indeed, eminently practical. What a difference, for example, there must be in the diagnosis, prognosis, and treatment of an attack of inflammation of the eye, in accordance with its causation by ordinary conditional influences (taking cold), by a particle of steel imbedded in the cornea, or by syphilis! How great the difference between the wound made by the teeth of an animal, in one case with, and in another without, the presence of rabies in its system! Take the instance of what we call fever: at a certain stage it is almost the same in half a dozen diseases. By the causation, when known, of this common congeries of symptoms we judge of the essential nature of the malady, and so of its proper treatment. It is a maxim in philosophy that every event or effect must have at least two causes. In medical etiology we often find many causes conspiring to produce one effect. These may be, and commonly have been, grouped together under two heads; as, 1, predisposing, and 2, exciting, causes. But under each of these may come a number of agencies contributing toward the production or modification of disease. Thus, of predisposing causes we may enumerate inherited constitution, habits of life, previous attacks of disease, atmosphere, and other immediate surroundings. Exciting causes--say, of an attack of apoplexy--may be, in the same case, mental shock, a stooping posture, an over-heated room, etc. One disease is very often the next preceding cause of another. So we speak of the great class of sequelæ of acute or subacute disorders; as, ophthalmia after measles, deafness following scarlet fever, or blindness small-pox, abscesses following typhoid fever, paralysis diphtheria, etc. But this kind of causation is extremely common also in chronic affections. What a train of organic troubles, of kidneys, heart, arteries, brain, and other parts, attend the affection to which we give the name of Bright's disease! How complex the sequence often of valvular disease of the heart, itself in many instances the effect of rheumatic fever, with {126} endocarditis as a local manifestation of that disorder! Hardly any discovery in pathology (or pathogeny, the generation of diseases) of the last half century has been more remarkable and fruitful than that of thrombosis and embolism, with their serious and not rarely fatal consequences, through obstruction of the blood-supply to different organs.