The reactionary results of injury to various tissues and the first local appearances due to the surgical infectious diseases are indicated by certain appearances, which, for a few hours at least, are in large measure common to both. Their beginnings being pathologically similar, their results depend not alone on the violence or intensity of the process, but also, and in predominating measure, upon the primary influences at work. The consequences of mere mechanical injury--such as strain, laceration, etc.--are in healthy individuals promptly repaired by processes which will be taken into consideration in the ensuing chapters. They are throughout conservative and reparative, and are directed toward restoring, as far as possible, the original condition. The consequences, on the other hand, of the surgical infections are more or less disastrous from the outset, although the extent of the disaster may be localized within a very small area, as after a trifling furuncle, or they may be so widespread as to disable a limb or an organ, or they may even be fatal. It is of the greatest importance, not alone for scientific reasons, but also because treatment must in large measure depend upon the underlying conditions, to differentiate between these two general classes of disturbance, which we speak of as-- A. _Those produced by external or extrinsic disturbances_, _i. e._, traumatisms, sprains, lacerations, etc.; and B. _Those produced by internal and intrinsic causes_, which, for the main part, are the now well-known microörganisms, such as cause the various surgical diseases. These latter disturbances may be imitated or _simulated_ in the presence of certain irritants within the tissues, such as the poisons of various insects and plants; the irritation produced by foreign bodies, minute or large; and possibly the presence within the system of certain poisons whose nature is not yet known, such as that of syphilis, or certain others whose chemistry is fairly well understood, but whose presence cannot be easily explained, as uric acid, etc. Clinically, all these disturbances are manifested by certain phenomena common to each, which may present themselves at one time more prominently, at another less so. These significant appearances have been recognized from time immemorial as the _calor_, _rubor_, _dolor_, _tumor_, _et functio lesa_ of our ancestors, or as the heat, redness, pain, swelling, and loss of function of our common experience. When one or more of these are present, the surgeon cannot afford to disregard the fact, while he should, moreover, be able to account for each on general principles which should to him be well known.