the invention will now be described in detail with reference to the foregoing figures where like numerals are used to designate like parts . in the foregoing discussion and elsewhere in the specification and appended claims , the terms &# 34 ; lower &# 34 ; and &# 34 ; downward &# 34 ; are intended to make reference to the needle end of the hypodermic syringes and associated parts described herein , and conversely the terms &# 34 ; upper &# 34 ; and &# 34 ; upward &# 34 ; are intended to make reference to the head end thereof . fig5 illustrates a cartridge ampoule , generally indicated by reference numeral 10 , of a well - known type which consists of a cylindrical container , usually glass or clear plastic , having a necked - down end and sealed at the necked - down end with a rubber diaphragm 11 which is secured to the ampoule by a crimped - on metal collar 12 . the other end of the ampoule is closed by a piston 13 which is slidable in the bore of the ampoule . the self - aspirating concept of the present invention is used in conjunction with syringe holders of the side loading type . one such embodiment , generally indicated by the reference numeral 14 , is illustrated in fig1 with details thereof shown in fig2 and 3 . the syringe there depicted has a hollow tubular body or barrel 15 having an elongated window 16 therein for insertion of a cartridge ampoule 10 , only the lower end of window 16 being depicted in the partial section view of fig1 . the syringe holder is fitted at its lower end with a needle hub 18 which is either detachably or integrally fitted with a hypodermic needle ( not shown ), the needle hub unit itself being detachably fitted to the syringe holder , for example by means of a screw - threaded mounting . the needle is of the double - ended type so that when a cartridge ampoule 10 is inserted in the syringe barrel 15 , the inner end of the needle pierces the diaphragm 11 so that the needle is in communication with the contents of the ampoule . the barrel 15 is attached to , and extends from , a head unit 19 to which is attached a pair of finger grips 20 . in the practice of the present invention , it is necessary that the ampoule 10 be essentially immobilized within the barrel 15 of the syringe holder , and accordingly for this purpose the head 19 is equipped with a locking sleeve 21 of generally cylindrical configuration , the shoulder 22 of which is biased downwards against the rim of the cartridge ampoule by compression spring 23 . slidably mounted within the bore of the locking sleeve is a double plunger which is composed of an inner plunger rod 24 which itself is slidable within the bore of a sleeve or outer plunger 25 . the inner plunger rod is fitted at its lower end with any conventional means for making positive interengagement with the rubber piston 13 in the cartridge ampoule 10 . for purposes of illustration , this interengagement means is depicted herein as a barbed point or &# 34 ; harpoon &# 34 ; 26 which is well known in the art for the stated purpose . however , it is to be understood that other conventional means of effecting interengagement between the plunger and the piston , such as those described in the prior art section above , will serve the purpose as well . the upper end of the inner plunger is fitted with a thumb plate 27 with a skirt 40 depending therefrom , and the inner plunger and outer plunger or sleeve are biased one against the other by a coil spring 28 . the lower end of the outer plunger or sleeve is fitted with an annular rim 29 which serves to prevent removal of the inner / outer plunger assembly from the bore of the locking sleeve 21 . the lower end of the inner plunger rod is also fitted with an annular rim 30 which serves to prevent removal of the inner plunger from within the bore of the sleeve or outer plunger . the thumb plate and skirt unit are threadably engaged with the upper end of the inner plunger . the threaded interengagement of the thumb plate and skirt unit with the plunger provides a means for assembling the inner / outer plunger unit within the head of the syringe . this assembly is accomplished by first removing the thumb plate / skirt / coil spring units 27 / 40 / 28 from the outer plunger , passing the inner and outer plungers , one within the other , through window 16 and upward through the bore of locking sleeve 21 and reassembling the coil spring and thumb plate / skirt units to the inner plunger . the locking sleeve 21 , at its widest diameter , is slidable within the bore of the syringe head 19 , and at its upper end has a section of diminished diameter which is slidable through the end opening of the syringe head and the opening through a locking nut 31 to be described hereinbelow . the upper end of the locking sleeve passes through a cavity 32 in the head of the syringe which is of sufficient diameter to accommodate a compression spring 23 which , as pointed out above , serves to bias the shoulder 22 of the locking sleeve 21 downwards against the rim of ampoule 10 . the upper end of the locking sleeve 21 having the smaller diameter is tapered as at 33 in fig2 and fig3 and is furthermore provided with slots 34 which form an equal number of flexible fingers 35 . the locking nut 31 mentioned above is fitted to the head of the syringe by means of threaded engagement 36 . the pitch of the threads is preferably selected so that the locking nut can be turned from its full up to its full down position in about one quarter turn of the nut . the locking nut is fitted with a thumb tab 37 to permit positive , selective and reversible manipulation of the locking nut by the operator as will be more fully described below , and the locking nut is prevented from being fully removed from the syringe head by an annular retaining flange 38 on the ends of the fingers 35 . the interior wall of the locking nut has a sloping surface 39 which is so configured that it generally conforms to the tapered surface 33 of locking sleeve 21 . thus with the locking nut in the full up or open position , the flexible fingers 35 are uncompressed , and the inner / outer plunger assembly 24 / 25 is freely slidable in the bore of the locking sleeve . however when the locking nut is turned to its full down or closed position , the sloping surface 39 of the locking nut slides downward over the mating tapered surface of the locking sleeve forcing the fingers inward against the outer plunger 25 thereby locking it against axial movement relative to the syringe holder . thus it is seen that the locking sleeve 21 serves a dual purpose as a locking sleeve to immobilize the cartridge ampoule in the syringe barrel and as a collet to positively , selectively and reversibly lock the outer plunger against axial movement . in use the syringe of fig1 is first loaded with a cartridge ampoule 10 by withdrawing the plunger / locking sleeve against the bias of compression spring 23 , inserting the ampoule through window 16 , and releasing the plunger / locking sleeve . engagement between the inner plunger rod 24 and the piston 13 is made , and in syringes having a detachable needle and needle hub unit , such unit is then attached . after air is expelled from the cartridge by downward pressure on thumb plate 27 , the needle is inserted in the injection site . when the operator wishes to aspirate to determine whether the needle tip has pierced a vein , he turns the locking nut 31 to its full down position by applying thumb pressure against thumb tab 37 thus securely locking the fingers 35 of the collet / locking sleeve 21 against the surface of outer plunger 25 and immobilizing the same against axial movement . thereafter when further downward pressure is exerted on thumb plate 27 , the inner plunger 24 advances beyond the outer plunger 25 as shown in fig4 . when pressure on the thumb plate is released , the inner plunger is withdrawn by the force generated by coil spring 28 , and the inner and outer plungers will once again assume the relative positions with respect to one another indicated in fig1 . as the inner plunger is withdrawn , the piston 13 to which it is firmly engaged via interengagement means 26 is likewise withdrawn slightly thus generating a slight negative pressure in the ampoule and creating aspirating conditions within the same . when the locking nut 31 is returned to its full up or open position by reverse manipulation of the thumb tab 37 , the collet is unlocked , and injection can thus be made by continued downward pressure on thumb plate 27 . it will be appreciated from the foregoing description that the self - aspirating syringe provided by the present invention possesses all the attributes of an ideal aspirating syringe as enumerated above . that is the syringe is relatively simple in construction , thus minimizing the cost of production ; it is relatively simple to operate ; it is capable of manipulation with one hand ; it is capable of multiple self - aspirating actions with each cartridge ampoule ; and it is capable of expelling trapped air from the ampoule either prior to initiation of the self - aspirating action or at any time during the sequence of actions necessary for injection of the ampoule contents without , on the one hand , precluding self - aspirating action at any point in the sequence or , on the other , rendering the self - aspirating action inoperative . moreover , the self - aspirating syringe of the invention can be constructed either in whole or in part from metal , to provide reusable units , or from plastic , to provide disposable units . it will also be understood that , although the preferred embodiments of the invention have been described above in order to better illustrate the same , alternative structural features can be substituted for elements described herein without either departing from the spirit of the invention or in any way adversely affecting the operability of the same . for example , as mentioned above , alternative conventional means of achieving interengagement between the inner plunger and the slidable ampoule piston can be used . furthermore , a thumb ring conventionally used in manually operating aspirating syringes , although not essential in the operation of the present automatic self - aspirating system , can nevertheless be used in place of a thumb plate . having thus described the invention and the advantages thereof , it is considered that the invention is to be broadly construed and limited only by the character of the following claims .
a hypodermic syringe of the automatic , or self - aspirating , type for use in combination with cartridge ampoules closed at one end by a diaphragm pierceable by a double - ended hypodermic needle and closed at the other end by a slidable piston consists of a syringe holder adapted to generate aspirating conditions within the cartridge ampoule by the slight backward displacement of the slidable piston by means of a double plunger , one slidable within , and biased against , the other , the inner plunger being positively interengaged with the slidable piston of the cartridge ampoule , and axial movement of the outer plunger being positively restricted by means for locking the other plunger with a collet in the end of the syringe holder .