conventional practice the scale 62 on the shield 37 may be preferable in this invention since the shield overlies the vial 11 also the vial liquid scale 35 shown in fig6 may sustain abrasion when the shield 37 is placed on the vial resulting in a scrap rate detrimental to manufacture on the other hand the liquid scale 62 could be misread due to refraction and this would indicate a preference for placing the scale on the vial 11 to be read through the clear plastics shield 37 either scale arrangement could be employed as found desirable fig4 shows the syringe 10 with the piston 59 fully retracted indicating that the syringe vial contains fluid fig5 shows the piston 59 thrust forwardly to the fluid unloading position the needle 12 and the entire device may now be contaminated thus far the shield 37 has no function and up to this point the syringe has been used in a conventional manner fig6 depicts the first functioning of the shield 37 finger flanges 50 and 50a are rotated counterclockwise from track 18 as shown in fig5 onto track 17 fig6 and in circumferential plane maintaining the flanges 50 and 50a in place until the surface 61 fig2 and inside partition chord 51a fig2 clear the forward stop 27 fig2 and 9 and the forward stop 27a fig1 rail notch 53 is disengaged from outside rail 19 in the relationship of rotation shown in fig6 rail notch 52 engages outside rail 20 to prevent override.