a hypodermic syringe of the automatic , or self - aspirating , type for use in combination with cartridge ampoules closed at one end by a diaphragm pierceable by a double - ended hypodermic needle and closed at the other end by a slidable piston consists of a syringe holder adapted to generate aspirating conditions within the cartridge ampoule by the slight backward displacement of the slidable piston by means of a double plunger , one slidable within , and biased against , the other , the inner plunger being positively interengaged with the slidable piston of the cartridge ampoule , and axial movement of the outer plunger being positively restricted by means for locking the other plunger with a collet in the end of the syringe holder .