some exemplary embodiments of the present invention relate to a syringe having a needle that is retractable and extendable such that the needle may be advanced through a series of specific needle positions , until the needle is fully extended . a carpule syringe facilitates repeated use , specifically repeated loading of carpules and enables easy insertion of doses of a medicament and discharge of exhausted containers . a carpule is a container or cartridge having a medicament therein . a carpule syringe is characterized by the absence of a fixed barrel in which a piston reciprocates . instead , a carpule syringe includes a shell with an opening designed to receive a medicament filled carpule , the ends of which are closed and hermetically sealed . the frame is provided with a reciprocatable shaft arranged to engage with one end of the carpule such that when the shaft is pushed forwardly in the frame , it will apply pressure to the liquid medicament within the carpule so as to dispense the same through a needle . referring to fig1 , an isometric view of an embodiment of a syringe assembly 10 with a needle 14 extended is generally shown . the syringe 10 is used in combination with a carpule 12 and a needle 14 . the carpule 12 is typically formed from glass or other transparent material is of conventional design well known in the art . carpule 12 is typically filled with a supply of fluid medication , such as novocaine , or the like . in one exemplary embodiment the outer shell 16 further comprises a window 20 to enable viewing of carpule 12 , and the quantity of medicament therein . referring to fig2 , an isometric view of an embodiment of a syringe 10 with a needle 14 retracted is generally shown . the syringe assembly 10 includes a hollow outer shell 16 that slidably engages at least a portion of an outer surface 21 of holder 18 and a plunger 24 slidably received in holder 18 . the outer shell 18 is movable between an extended position shown in fig1 and a retracted position shown in fig2 for exposing the needle 14 . in a further exemplary embodiment , the plunger 24 further includes a gauge for indicating the quantity of medicament in the carpule 12 . fig3 illustrates an isometric view of an exemplary embodiment of a portion of the syringe 10 having the outer shell 16 removed . the holder 18 includes an aperture 15 for receiving the carpule 12 therein . a distal end 22 of the holder 18 has an aperture configured to receive a needle 14 . a portion of plunger 24 extends outwardly from the proximal end 26 of the holder 18 . referring to fig4 , a portion of the syringe assembly 10 is generally shown . the outer surface 21 of holder 18 includes at least one depression 31 , 32 , 35 , 36 for selectively engaging a protrusion 50 located at a proximal end 48 of the outer shell 16 . in one exemplary embodiment , the protrusion 50 is formed on the inner wall 28 of the outer shell 16 . the outer surface 21 of holder 18 includes a first depression 36 that defines a first position 38 , and a second depression 32 that defines a second position 34 . the second depression 32 is proximate to the proximal end 26 of holder 18 , and the first depression 36 is distal to the second depression 32 . the distance between the first depression 36 and the second depression 32 , and any intermediary depressions , determines the relationship of the needle 14 relative to the outer shell 16 . in another exemplary embodiment , the outer surface 21 of holder 18 further includes two intermediate depressions for selectively engaging the protrusion 50 of outer shell 16 . the first intermediate depression 31 defines a first intermediate position 33 and the second intermediate depression 35 defines a second intermediate position 37 . the second intermediate depression 35 is proximate to the second depression 32 and the first intermediate depression 31 is distal to each of the second intermediate depression 35 and the second depression 32 . the outer shell 16 in an extended position and the second position 34 corresponds to the outer shell 16 in a retracted position . protrusion 50 of outer shell 18 is movable between the first 38 and second 34 positions . the first position 38 corresponds to the outer shell 16 in an extended position and the second position 34 corresponds to the outer shell 16 in a retracted position . in another exemplary embodiment , the first intermediate position 33 and second intermediate position 37 both correspond to the outer shell 16 in a partially retracted position where only a portion of needle 14 is visible . when protrusion 50 is engaged in the second intermediate depression 35 , a greater portion of needle 14 is visible compared to when protrusion 50 is engaged in the first intermediate depression 31 . referring to fig5 , a top view , partially in section , of the syringe assembly 10 is generally shown . the syringe assembly 10 further includes a spring 23 biased toward a forward end 25 of plunger 24 . this forward end 25 of plunger 24 is biased forwardly by spring 23 to resiliently hold carpule 12 in holder 18 . in one embodiment , the syringe assembly 10 is self - aspirating . referring to fig6 , a partial sectional view of the syringe assembly 10 is generally shown . the holder 18 further includes a channel 40 for continuously engaging protrusion 50 of outer shell 16 . the channel 40 is formed longitudinally along at least a portion of the length of holder 18 . in one embodiment , the outer shell 16 further includes at least one protuberance 27 engaging channel 40 . the engagement of protuberance 27 in channel 40 prevents the rotation of holder 18 within outer shell 16 . in another embodiment , the first depression 36 and second depression 32 are formed in channel 40 . in a further embodiment , the outer shell further includes a grip 17 that covers a portion of the proximal end 48 of the outer wall 29 of the outer shell 16 for ease of holding the syringe assembly 10 by a user . referring to fig7 , an isometric view of a portion of an embodiment of the syringe assembly showing a section of an inner wall of the outer shell is generally shown . a gap 19 is provided at the proximal end 48 of outer shell 16 to allow for expansion of the outer shell 16 . the outer shell further comprising an opening 47 for receiving a needle 14 . in a further embodiment , the inner surface 28 of outer shell 16 including a conduit 60 extending from the proximal end 48 to the carpule 12 for allowing self - aspiration of the syringe assembly 10 . in one embodiment , a portion of a distal end 47 of outer shell 16 is textured to obscure viewing of needle 14 . in another embodiment , a portion of the proximal end 48 of outer shell 16 is coated or otherwise covered with a polymeric material that is similar to that material found on toothbrushes . in another embodiment , the syringe assembly of the present invention can be manufactured by methods known to those skilled in these arts . more particularly , the outer shell 16 can be injection molded in two identical pieces that are then joined together or in a single mold . in a further embodiment , each of holder 18 and plunger 24 may also be injection molded . in one embodiment , the outer shell 16 is constructed from suitable flexible materials such as plastics , metals , or other materials known in the art that . the proximal end 48 of the outer shell 16 may be temporarily deformed to enable the proximal end 48 to be biased toward the holder 18 . the syringe assembly 10 to load the syringe assembly 10 with a carpule , a user pulls plunger 24 away from the proximal end 48 of outer shell 16 such that the plunger 24 is fully extended , spring 23 is in tension , and aperture 15 of holder 18 is exposed for loading a carpule 12 in holder 18 . once the carpule is loaded , the plunger 24 slides forward toward the proximal end 48 of outer shell 16 , spring 23 is biased toward the carpule holding the carpule resiliently in place in holder 18 , and the protrusion 50 is engaged in the first depression 36 . in another embodiment , the syringe assembly 10 may be loaded with a carpule using methods known to persons skilled in the art . in use , the syringe assembly once loaded with a carpule 12 , the outer shell 16 is in an extended position such that needle 14 is in a retracted position and is completed covered by the outer shell 16 as shown in fig2 , and the protrusion 50 of outer shell 16 is engaged in the first depression 36 of holder 18 . this is the first position 38 . the user then applies pressure to the plunger 24 . the plunger 24 and holder 18 move as a unit relative to outer shell 16 , and the protrusion 50 springs out of the first depression 36 , and the holder 18 and plunger 24 together slidably engage the outer shell 16 sliding toward the distal end of outer shell 16 until protrusion 50 engages a first intermediate depression 31 . in this first intermediate position 33 a portion of needle 14 is visible and the outer shell 16 is partially retracted . pressure continues to be applied by the user and protrusion 50 springs out of the first intermediate depression 31 , the holder 18 and plunger 24 continue to slide as a unit in the outer shell 16 until protrusion 50 engages a second intermediate depression 35 . in this second intermediate position 37 , a greater portion of needle 14 is visible and a greater portion of the outer shell 16 is partially retracted . pressure continues to be applied by the user and protrusion 50 springs out of the second intermediate depression 35 , the holder 18 and plunger 24 together continue to slide as a unit in outer shell 16 until protrusion 50 then engages the second depression 32 . in this second position 34 , the outer shell 16 is in a retracted position such that needle 14 is in an extended position as shown in fig1 . upon reaching the second depression , the user applies further pressure to plunger 24 . the plunger 24 slides in holder 18 and the forward end 25 of the plunger 24 selectively engages the container 12 . the continued pressure by the uses moves the plunger 24 to a dispensing position and urges the contents of the carpule 12 from the carpule 12 , through the needle 14 . the above - described embodiments have been provided as examples , for clarity in understanding the invention . a person of skill in the art will recognize that alterations , modifications and variations may be effected to the embodiments described above while remaining within the scope of the invention as defined by the claims appended hereto .