Outlines of Pathology And Practice of Medicine

that certain circumstances in the situation or condition of patients, before diseases appear, can be assigned with confidence as their causes. The efficacy of these, however, is seldom established in any other way than simply by the observation, that persons known to be exposed to their influence, become afflicted with certain diseases in a proportion very much greater than those who are not known to be so exposed. This kind of evidence is in many individual cases very liable to fallacy, in consequence of the great variety of the circumstances, capable of affecting health, in which individuals are placed, and of the difficulty of varying these, Po as to obtain such observations, in the way of induction, or exclusion, as shall be decisive as to the efficacy of each. Hence the importance of the observations, intended to illustrate this matter, being as extensively multiplied as possible; and hence also the peculiar value, with a view to the investigation of the causes of diseases, of observations made on large and organized bodies of men, as in the experience of military and naval practitioners. All the circumstances of the whole number of men, whose diseases are there observed, are in many respects exactly alike ; they are accurately known to the observer, and are indeed often to a certain degree at his disposal; they are often suddenly changed, and when changed as to one portion of the individuals under observation, they are often unchanged as to another; and t...