referring first to fig1 the illustrative preferred embodiment of the invention comprises a hollow fluid injection needle 10 , injection molded into a generally transparent plastic syringe barrel 12 , with graduated dosage indicia 14 , and finger flanges 16 , all of which are essentially the same as with today &# 39 ; s state of the art products . in addition the barrel comprises : a used needle warning 18 , which circumscribes barrel 12 ; annular tooth retaining groove 20 which circumscribes the needle end of barrel 12 ; and a annular detent groove 22 including a sloped surface 23 which circumscribes the barrel 12 . fig2 shows a plunger assembly comprised of a plunger shaft 24 , with a resilient plunger seal 26 , and a thumb pad 28 . it is identical to an assembly used in one of today &# 39 ; s state of the art disposable syringes . fig3 shows a needle guard or cap 30 . unlike conventional caps , cap 30 includes a particular bullet like or tapered surface 31 , a major advantage of which is explained later in connection with assembly procedures , in addition to an open entry end 43 . fig4 shows a unitary sliding cylinder or sleeve 32 , which is also a generally transparent plastic part , and which incorporates at least one radially inward facing latching pawl 34 bearing a resiliently supported pawl tooth 35 , supported by a resilient pawl arm 36 . an externally accessible roughened pawl grasping surface 38 , is provided on the free end of the pawl arm 36 radially opposite the tooth 35 . additionally another , separate roughened sleeve grasping surface 40 circumscribes one end of the sleeve 32 ; and finally a opaque band 41 circumscribes the opposite end of the sleeve 32 , as a means of obscuring from sight anything underlying it . the sleeve 32 can be injection molded from a thermoplastic material possessing qualities of transparency , hardness , sharp detail , and springiness , with memory to return to it &# 39 ; s molded shape . to initiate assembly of the foregoing four parts , the plunger assembly of fig2 is first fully seated into the internal cavity 42 of the syringe barrel 12 of fig1 thereby purging air from the internal cavity 42 of the barrel 12 out through the hollow needle 10 . then , in preparation for the assembly of the sleeve 32 onto barrel 12 , the cap 30 is fitted onto the needle end of barrel 12 , thereby sheathing the needle 10 . finally , the sleeve 32 is then slid past cap 30 , which due to it &# 39 ; s tapered surface 31 , cams the radially inward facing pawl tooth 34 outward from the sleeve &# 39 ; s central axis , allowing it to pass over the annular tooth retaining groove 20 . sleeve 32 is slid fully onto barrel 12 , into a yieldable retracted position as shown in fig5 ; thereby hiding the narrower warning band 18 beneath the wider opaque band 41 , and seating a latching tooth 35 resiliently into the annular detent groove 22 . the term yieldable means that sleeve 32 is held in the assembled , retracted position shown in fig5 by means of detent and / or friction , but that it can later be slid relative to the syringe barrel 12 toward it &# 39 ; s extended position as shown in fig7 by user application of a relatively moderate axial force , without requiring special releasing or unlatching activity . this resistance to axial motion is sufficient to maintain the retracted position of sleeve 32 during routine handling of the syringe . because all parts are generally cylindrical , and because grooves 20 and 22 circumscribe barrel 12 , no rotational orientation is required between any of the parts at any time . the procedure for use of the syringe , to inject medication into a patient as an example , is as follows : first , the sterile syringe is removed from it &# 39 ; s protective wrapping , not shown . then the needle cap 30 is removed from it &# 39 ; s protective initial position and set aside for later use . although this step exposes the user to possible injury by needle 10 for the first time , such an injury would pose no significant health threat as the needle 10 would not have yet been exposed to potential contamination . next , the user , grasps the syringe sleeve 32 between thumb and forefinger of one hand with either the thumb or the forefinger resting over the pawl grasping surface 38 , and squeezes thereby seating tooth 35 firmly into the annular detent groove 22 . this measure , which will henceforth be repeated each time the syringe is handled during a critical procedure , will provide the user with positive means for preventing premature extension of the sleeve 32 from it &# 39 ; s retracted position . inadvertent stray overlap of the thumb or forefinger from the pawl grasping surface to the sleeve surface will do not harm , as any distortion of the resilient nature of the sleeve material will merely serve to enhance the frictional resistance which occurs naturally between the sleeve and the barrel , thereby furthering the induced opposition to a premature extension of the sleeve . the user will next withdraw the plunger assembly with the free hand , by means of it &# 39 ; s thumb pad 28 , to a position corresponding to the volume of the intended dose of medicine . calibrated dosage indicia 14 , provided on the exterior wall of the syringe barrel , may be observed through the transparent portions of sleeve 32 . the action thus described draws air into the internal cavity 42 of the barrel 12 , equal in volume to the intended dosage . the syringe now appears as in fig6 . then , while still , or yet again , maintaining preventive measures opposing a premature extension of the sleeve 32 , the user inserts the needle 10 into the medication bottle , not shown , and injects the entrapped air into the bottle by squeezing the thumb pad 28 toward the barrel &# 39 ; s finger flanges 16 in the conventional way . thus the amount of air , necessary to replace the intended withdrawal of medicine , has been placed in the bottle . the user next loads the syringe with the prescribed dose of medicine as follows : with one hand the user prevents premature extension of the sleeve 32 as was explained above , and withdraws the plunger assembly of fig2 the required distance out of the barrel &# 39 ; s internal cavity 42 as seen in fig6 by means of pulling on the thumb pad 28 with the thumb and forefinger of the free hand . thus , with the syringe properly charged with the appropriate dose of medicine , the syringe is withdrawn from the bottle of medicine and with needle 10 pointed upward , any and all possible remaining air is purged from the syringe barrel 12 and / or the needle 10 , in accord with conventional medical procedure . with the syringe thus configured as in fig6 ready for the injection of medicine into a patient , and with firm user interaction to prevent a premature extension of the sleeve 32 , as discussed above , the needle 10 is inserted into the body tissue of the patient , the syringe normally aspirated , and the medicine is injected into the patient &# 39 ; s body tissues , in the normal way , using the thumb , forefinger , and middle finger of either hand . following the injection , the user next holds the sleeve 32 , without squeezing between the thumb and forefinger of the free hand , at any convenient location on sleeve 32 except coincident with the pawl grasping surface 38 thereby restraining the sleeve 32 against the patient &# 39 ; s skin , not shown , and , by means of the finger flanges 16 already in appropriate contact with the pads of the index and middle fingers of the other hand , safely withdraws the needle 10 from the patient . because sleeve 32 is held in it &# 39 ; s position against the patient &# 39 ; s skin while the remainder of the syringe is being retracted , the needle 10 becomes enshrouded within the protective confines of sleeve 32 . the resiliently supported pawl tooth 35 , having been cammed out of the releasable annular detent groove 22 due to the grooves sloped surface 23 and because the user has applied the appropriate axial forces , slides along the surface of the barrel 12 , and snaps into a permanently locked engagement with the annular tooth retaining groove 20 , thereby securing the needle 10 from all further potential for accidental contact . should the user &# 39 ; s grasp on the sleeve 32 slip inadvertently during the attempt to extend the sleeve 32 , sleeve grasping surface 40 , is in place , and is of saw tooth shape so as to terminally oppose continued slippage of the user &# 39 ; s grasp completely off of the end of the sleeve 32 . the needle 10 is thus safely enshrouded as per fig7 within the extended sliding sleeve 32 . note that due to the full 360 degree circumferential nature of the latching arrangement , the latching engagement takes place regardless of the relative rotational orientation between the barrel 12 , and the sliding sleeve 32 . equally important is the fact that after sleeve 32 and barrel 12 are latched together by means of resiliently supported pawl the seating of tooth 35 into annular tooth retaining groove 20 , sufficient overlap 44 is provided between these two members to assure a structural integrity capable of withstanding considerable forces to which the syringe might be subjected before it &# 39 ; s disposal . additionally , it should be noted that simultaneously with withdrawal of the needle 10 from the patient , and the simultaneous extension of sleeve 32 the warning 18 ( red as an example ), previously hidden by the wider circumferential obscurring opaque band 41 ( yellow as an example ), is exposed to view . finally , the needle cap 30 which had been set aside earlier , is retrieved by the user , and placed over the point of the needle 10 , and within the extended protective sleeve 32 , so as to remove all further threat of blood , body fluids , or tissues being accidently dispersed , ( for example in the case where a needle should be accidentally dropped ). this placement of the cap 30 to a position just short of it &# 39 ; s initially assembled position , as best shown in fig8 will now be possible to do with no danger of injury because the needle 10 has already been hidden from harms way within the protective sleeve 32 . further this placement of the cap 30 over the needle 10 cannot function to release resiliently supported pawl tooth 35 from it &# 39 ; s latched relationship with the annular tooth retaining groove 20 because the cap &# 39 ; s open entry end 43 squarely encounters the arresting face of the seated resiliently supported pawl tooth 35 . the used syringe is now ready for appropriate disposal . the alternate preferred embodiment of the present invention , shown in it &# 39 ; s initial configuration in fig9 is similar to the previously discussed preferred embodiment except that it uses at least two pawls rather than at least one ; with extension arresting pawl 45 resisting extension of the sleeve 32 &# 39 ; and with latching pawl 47 resisting the sleeve &# 39 ; s motion onto the barrel 12 &# 39 ;. this arrangement removes the need to provide the circumferential pawl retaining groove 20 of the preferred embodiment ( shown in fig1 ), but does require a annular extension arresting groove 46 to be molded into the surface of the syringe barrel 12 &# 39 ; instead . this annular extension arresting groove 46 will receive extension arresting pawl 45 as shown in fig1 , so as to terminate the extension of sleeve 32 &# 39 ; as the needle is withdrawn from the patient back into the protection of the extending sleeve 32 &# 39 ; ( similarly as shown in fig1 but without cap 30 &# 39 ;). as can best be seen in fig9 because of the location of the arresting groove 46 on barrel 12 &# 39 ;, it also serves as a means of detent for latching pawl 47 , to yieldably resist premature extension of the sleeve 32 &# 39 ;, from the initially retracted or assembled position . similarly the annular detent groove 48 , surrounding barrel 12 &# 39 ; will provide additional resistance when sleeve 32 &# 39 ; is in it &# 39 ; s retracted position , as a result of it &# 39 ; s yieldable relationship with extension arresting pawl 45 . the roughened pawl surface 50 and the shape of latching pawl 47 will protrude radially beyond the exterior cylindrical surface of the sleeve 32 &# 39 ; when the sleeve 32 &# 39 ; is in it &# 39 ; s initially retracted position , so that the latching pawl 47 can be squeezed by the user , through use of the thumb and forefinger of one hand , in a manner similar to that explained at length for the previously discussed preferred embodiment , thereby providing the user with the same positive control to prevent premature extension of the sleeve 32 &# 39 ;. inadvertent stray overlap of the thumb or forefinger from the latching pawl 47 , roughened pawl surface 50 , to the surface of sleeve 32 &# 39 ; will do no harm , as any resulting distortion of the resilient nature of the material of sleeve 32 &# 39 ; will merely serve to enhance the frictional resistance which occurs naturally between the sleeve 32 &# 39 ; and the barrel 12 &# 39 ;, thereby furthering the induced opposition to a premature extension of the sleeve 32 &# 39 ;. as can best be seen in fig1 , upon full extension of the sleeve 32 &# 39 ;, as extension arresting 45 engages with annular extension arresting groove 46 and latching pawl 47 snaps radially inward off the needle barrel end corner 52 of the syringe barrel 12 &# 39 ;, preventing the return of the sleeve 32 &# 39 ; onto the barrel 12 &# 39 ;. in examining the after use configuration , shown in fig1 , it becomes clear that the inner surface of the resilient arm that carries latching pawl 47 must have a step or notch 54 , formed into it so that any effort to return the cap 30 &# 39 ; to a position further isolating needle 10 &# 39 ;, can not undesirably cam the latching pawl 47 radially outward thereby enabling the sleeve 32 &# 39 ; to be returned to it &# 39 ; s initially retracted position so that the syringe could be used a second time . the invention has been described in detail with particular reference to preferred embodiments thereof , but it will be understood that variations and modifications can be effected within the spirit and scope of the invention . for example , it should be understood that while the invention is described in terms of a hypodermic syringe being used to inject medication into body tissue , the invention also encompasses the use of the syringe for removing blood or other body fluids .