referring to fig1 there is shown a section elevation view of the syringe 1 of the preferred embodiment . the syringe 1 is comprised of an elongated hollow barrel 2 which is a round tube like in configuration with a first end and a second end and with an inner surface 21 and an outer surface 22 . inside of the elongated hollow barrel 2 is the slidable plunger assembly 3 . the slidable plunger 3 with a first end and a second end has a thumb flat 4 at the second end and a plunger piston 23 at the first end . the thumb flat 4 is fixed to the plunger piston 23 by the plunger rod 24 . the plunger piston 23 also has a plunger gasket 9 which forms a liquid tight and gas tight seal between the plunger piston 23 and the inner surface 21 of the elongated hollow barrel 2 . the elongated hollow barrel 2 has a longitudinal axis 28 in the center of the elongated hollow barrel 2 . the longitudinal axis 28 has a first end and a second end and runs the entire length of the elongated hollow barrel 2 from the thumb flat 4 to the needle cannula 5 . also shown inside of the elongated hollow barrel 2 is the slidable piston 6 . the slidable piston 6 is fixed to the needle cannula 5 . at the second end of the slidable piston 6 is a pimple 25 which forms part of the slidable piston 6 . the slidable piston 6 is shown held in place within the elongated hollow barrel 2 by the trigger 10 and the piston gasket 8 . the piston gasket 8 is fixed to the elongated hollow barrel 2 but it could also be fixed to the slidable piston by design choice . the slidable piston 6 is further held in place at the first end by a partly compressed spring 7 . the partly compressed spring 7 has a first end resting on the barrel flange 26 and a second end integral with the piston flange 15 . the partly compressed spring 7 is pushing on the piston flange 15 of the slidable piston 6 . the slidable piston 6 is held and fixed in place by the slidable trigger 10 on each side of the elongated hollow barrel 2 . the syringe 1 is shown held between finger 31 and finger 32 and it is further held and restrained from moving between finger 31 and finger 32 by finger stops 33 . the slidable plunger assembly 3 is depressed or pushed by pressure from the thumb 30 on the thumb flat 4 . the slidable plunger assembly 3 is shown moving in a needleward direction 20 forcing medicament 27 through the cannula 29 in the slidable piston 6 and further through the needle cannula 5 into a body not shown or part of a body not shown . the needle cannula 5 is shown with a first end and second end , the first end has a point that will enter a body and the second end is shown fixed to the slidable piston 6 . the spring 7 is shown only partly compressed , with the first end of the spring 7 resting on the barrel flange 26 and the second end of the spring 7 pushing on the piston flange 15 . the slidable piston 6 and needle cannula 5 are held firmly and rigidly in the elongated hollow barrel 2 by the combination of the inner rim 13 and the outer rim 34 supporting the needle cannula 5 and the piston gasket 8 supporting the slidable piston 6 ; this prevents movement of the slidable piston 6 and the needle cannula 5 in a direction perpendicular to the longitudinal axis 28 of the elongated hollow barrel 2 . the slidable piston 6 and the needle cannula 5 are also held firmly and rigidly within the elongated hollow barrel 2 by the spring 7 that is partly compressed and is thrusting against the barrel flange 26 and the piston flange 15 in combination with the one or two slidable triggers 10 that restrain the spring 7 from thrusting the slidable piston assembly 6 and needle cannula 5 in a thumbward direction 19 . the slidable trigger 10 has a first end and a second end and is held in place at the first end and supported within the elongated hollow barrel 2 by a left guide 16 and right guide 17 . the slidable trigger 10 extends from the left guide 16 and the right guide 17 of the elongated hollow barrel 2 into the trigger notch 12 that is cut into the slidable piston flange 15 of the slidable piston 6 . the slidable trigger 10 is further shown with a trigger slope 11 that slopes in an outward direction 18 and a thumb ward direction 9 . the trigger notch 12 has a cut that is perpendicular to the longitudinal axis 28 of the elongated hollow barrel 2 and it also has a trigger notch slope 14 that is parallel to the trigger slope 11 on the second end of the slidable trigger 10 . the slidable trigger 10 is held in place within the elongated hollow barrel 2 and the trigger notch 12 by friction or other suitable mean . referring to fig2 there is shown a section elevation of the preferred embodiment of the present invention . the medicament has already been discharged and the needle cannula 5 has been withdrawn from the body not shown or part of the body not shown . the slidable plunger assembly 3 is pushed further in a needle ward direction 20 by pressure from the thumb 30 , on the slidable plunger assembly 3 further pushing the slidable piston 6 and needle cannula 5 in a needle ward direction 20 , further compressing the spring 7 . as the slidable piston 6 is moved in a needle ward direction 20 , the trigger notch slips 14 of the trigger notch 12 reacts with the trigger slope 11 of the sliding trigger 10 thus forcing the sliding trigger 10 to move in an outward direction 18 , that is a direction perpendicular to the longitudinal axis 28 of the elongated hollow barrel 2 . the piston gasket 8 is shown compressed by the plunger piston 23 . as the slidable trigger 10 is moved in an outward direction 18 , it will either fall out or be held in place within the elongated hollow barrel by friction , which is a matter of design choice . referring to fig3 there is shown a section elevation of the preferred embodiment of the present invention . the slidable trigger 10 has been moved out of the trigger notch 12 and the thumb shown in fig1 is removed from the thumb flat 4 thus allowing the spring 7 to push the slidable piston 6 and the needle cannula 5 into the elongated hollow barrel 2 , where the needle cannula 5 will be completely enclosed by the elongated hollow barrel 2 the barrel flange 26 and the inner rim 13 . the spring 7 may have sufficient force to push the slidable plunger assembly 3 partly out of the hollow elongated barrel 2 of the syringe assembly 1 . with the spring 7 constantly pushing or thrusting the slidable piston 6 in a thumbward direction inside the hollow elongated barrel 2 , the pimple 25 on the slidable piston 6 forces the slidable piston 6 and the needle cannula 5 to slant relative to the longitudinal axis 28 thereby moving the tip of the needle cannula 5 away from inner rim 13 and the needle cannula tunnel 35 whereby the needle cannula 5 cannot fall to a position where the point of the needle cannula 5 cannot be pushed outside of the confines of the hollow elongated barrel 2 , the barrel flange 26 and the inner rim 13 . referring to fig4 there is shown a section elevation view of the preferred embodiment as taken through fig1 . the slidable trigger 10 is shown in the trigger notch 12 of the slidable piston 6 . the slidable trigger 10 is also shown in the trigger slot 36 of the elongated hollow barrel 2 . referring to fig5 there is shown a section elevation view of the first preferred embodiment as taken though fig2 . the slidable trigger 10 is shown pushed out from the trigger notch 12 of the slidable piston 6 . the slidable trigger 10 is also shown projecting out of the elongated hollow barrel 2 of the syringe 1 . referring to fig6 there is shown a partial elevation of the outer surface 22 of the hollow elongated barrel 2 . the slidable trigger 10 is shown as being square however it could also be round or rectangular by design choice . the left guide 16 and the right guide 17 are shown with hidden lines . although the system described in detail supra has been found to be most satisfactory and preferred many variations are possible . for example the syringe may have three or four triggers , the syringe could be square in section or the trigger could be placed closer to the needle . although the invention has been described with reference to the preferred embodiment , it will be understood by those skilled in the art , that additions , modifications , substitutions , deletions and other changes not specifically described , may be made in the embodiments herein , it should be understood that the details herein are to be interpreted as illustrative and are not in a limiting sense .
a disposable hypodermic syringe having a needle cannula fixed to a piston assembly . the piston assembly is held within the elongated hollow barrel of said hypodermic syringe by a compressed spring and a trigger . when said trigger is disengaged with said piston assembly said spring pushes said piston assembly and said needle cannula into said barrel of said hypodermic syringe and holds said needle cannula fixed to said piston assembly within said barrel and is further prevented from being pushed out of said elongated hollow barrel by a rim in said elongated hollow barrel thus preventing any accidental injection of bacteria , virus or other undesirable material into others . the disengagement of said trigger with said piston assembly is accomplished with only one and the same hand that is used to inject the needle cannula and medicament into a body .