It is seldom that a superficial collection of pus can be mistaken for anything else. In small and superficial abscesses (boils, furuncles) as pus approaches the superficial layer (epidermis) of the skin it may be discovered through its thin covering. In deep lesions there is often a doubt, even on the part of the most experienced. The measure now usually resorted to for purposes of diagnosis and exact recognition is the exploring or aspirating needle. The old exploring needle was one of good size, having a groove along which, after introduction, pus might pass. Since the almost universal use of the hypodermic syringe, a small aspirating needle attached to the ordinary syringe is the measure commonly adopted. Such a needle may be introduced into the brain, into the liver, or into almost any and every soft tissue without danger, and if properly manipulated is almost sure to facilitate detection of pus. Exploration done with either of these means and for this purpose should always be conducted as an aseptic, even if a minor operation, in order that no extra infection may be added from without. The skin should be carefully washed, the needle sterilized, etc. It is good surgery to resort to the knife either for the above purpose or in order that by a longer incision or by opening the cavity deep exploration may be made. Such explorations are of benefit even though a circumscribed collection of pus is not found, since by relief of tension and local abstraction of blood they act in a revulsive way and do much good. Acting upon the same principle the trephine or the bone chisel may be used for the purpose of opening the cranium and exploring for pus, or of opening into the medullary canal of the long bones and hunting there for that which is suspected.