pull out the needle from the skin there is no chance for air to go into the artillery or human tissue even when an injector carelessly pushed the plunger and the hub 11 all the way down to the bottom through the bore 51 although it is very unlikely to happen because the hub 11 is elliptical in shape there is still room for the residual air if there is this can hardly be seen from the drawing in fig3 since the drawing is to indicate the engagement mechanism of the hub with the carrier the room allocated for the accommodation of the residual air if any can better be understood from a top view not shown instead of the side cross sectional view as the plunger retracts the carrier with the needle into the barrel and come to a stop and break the detailed drawing of the cross sections are shown in fig4 while the plunger pulls backwards the carrier and the elliptical disk 14 is in contact with the bore 311 on the inner wall at the upper end of the barrel the plunger can be easily broken off at the fragile section between parts 14 which is a solid elliptical piece and 16 no matter what angle around the shaft is the bending force applied onto.