a syringe in which a piston is releasably latched to a plunger for reciprocation in a cylindrical barrel , wherby the piston will become disengaged from the plunger upon one cycle of rearward and then forward movement of the plunger and piston in the barrel , followed by subsequent rearward movement of the plunger in the barrel , thus rendering the syringe incapable of reuse . the piston is held to the plunger by latching arms which move to an over - center position between their latched and unlatched positions .