This instrument was devised with a view to prevent accidents in the opening of the cavities of the antrum, the chest and cysts, hydroceles and large abscesses. There is always danger when puncturing the antrum of Highmore, the thorax and other cavities which are filled with fluid abnormal in character and which must be evacuated of the trocar under the force sometimes necessary to puncture the tissues forming the walls of these cavities, injuring the organs contained within them, or of passing through the opposite wall and causing injuries to structures beyond. To render such accidents impossible this instrument has been fitted with a safety guard entering the handle of the instrument by means of a screw thread which makes it possible for the operator to set the guard so that the trocar and canula will enter at any depth that he may judge will be the thickness of the