A device for locating a cavity within two walls including a syringe including a syringe piston slideable in a first syringe barrel for discharging air or fluid contained in the first syringe barrel on forward movement of the syringe piston into the cavity, the first syringe barrel being attached to a hub of a needle cannula and a frame which at least in part encases the syringe and which is solely connected to the back of the syringe piston.
