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INCORPORATION BY REFERENCE 
     The present application is a continuation of U.S. patent application Ser. No. 12/441,179, filed Mar. 13, 2009, the entire contents of which are incorporated herein by reference as if fully set forth. 
    
    
     BACKGROUND 
     The invention relates to a jet regulator which can be inserted into the water outlet of a sanitary outlet fitting from the direction of the outlet end side, with it being possible for the jet regulator to be detachably fastened or fixed in the water outlet. 
     It is already known for a jet regulator which is to form a homogenous, non-sputtering water jet to be mounted on the water outlet of a sanitary outlet fitting. Such jet regulators are usually inserted into an outlet mouthpiece which can be detachably screwed to the water outlet of the sanitary outlet fitting. 
     Since the configuration of the outlet mouthpiece in designing the surface of the sanitary outlet fitting may involve a considerable amount of expenditure, and since the gap remaining between the outlet mouthpiece and the outlet fitting is often perceived as a problem, jet regulators of the type mentioned in the introduction have already been created which can be inserted into the water outlet of a sanitary outlet fitting from the direction of the outlet end side without an additional outlet mouthpiece being necessary for fastening the jet regulator. 
     DE-A-31 14 818 has disclosed a jet regulator which can be inserted into the water outlet of a sanitary outlet fitting. To be able to insert the previously disclosed jet regulator into the water outlet of the outlet fitting, a screw head cap which serves as an outlet mouthpiece is provided on the water outlet, which screw head cap can be screwed by means of a peripheral external thread onto an internal thread which is provided on the inner periphery of the adjacent face-end region of the outlet fitting. The jet regulator previously disclosed by DE-A-31 14 818 is provided as an aerated jet regulator which, for the induction of the air which is to be mixed with the water jet, has air admixture openings which are arranged so as to be distributed about the periphery of its jet regulator housing. Here, an encircling annular gap is provided between the jet regulator housing and the adjacent inner wall of the screw head cap which serves as an outlet mouthpiece, via which annular gap the air which is inducted into the water outlet at the end side can enter into the housing interior of the jet regulator via the air admixture openings which are provided in the jet regulator housing. The jet regulator previously disclosed by DE-A-31 14 818 is inserted into the inflow side insertion opening of the outlet mouthpiece which is designed as a screw head cap and which itself can be screwed to the water outlet of the outlet fitting. Here, the jet regulator previously disclosed by DE-A-31 14 818 has an aeration duct which is designed as an annular gap which runs around the jet regulator housing at all sides. Since the jet regulator housing of the previously disclosed jet regulator surrounds an encircling annular gap, the cross section of the emerging water jet is comparatively small in relation to the outer periphery of the water outlet. 
     WO-A-98/16695 has already disclosed a jet regulator having a jet diffuser device and a jet regulating device which is connected downstream of said jet diffuser device in the flow direction, with the jet regulating device which is provided in the jet regulator housing of the previously disclosed jet regulator having a plurality of pin-like or annular impact bodies which are spaced apart from one another and which are arranged in the flow path transversely with respect to the flow direction. In all of the exemplary embodiments illustrated in WO-A-98/16695, the jet regulator previously disclosed there has an annular flange which is arranged at the inflow side and which projects beyond the periphery of the jet regulator housing, by means of which annular flange the jet regulator housing can be placed onto the annular shoulder provided at the inner periphery of an outlet mouthpiece. On the jet regulator previously disclosed by WO-A-98/16695 is therefore mounted by means of a conventional outlet mouthpiece on the face-end region, which is provided as a water outlet, of a sanitary outlet fitting. Here, in said previously disclosed jet regulator too, an annular gap is provided between the jet regulator housing and the adjacent inner periphery of the outlet mouthpiece in order that the air for admixture can be inducted into the housing interior of the jet regulator housing via the outlet-side face end of the outlet fitting and the air admixture openings which are distributed about the outer periphery of the jet regulator housing. The jet regulator previously disclosed by WO-A-98/16695 therefore also has the disadvantages already mentioned above with regard to the prior art. 
     SUMMARY 
     With the ever-increasing aesthetic demands on sanitary outlet fittings, the demands made of the jet regulators required also increase. It is therefore the object to create a jet regulator of the type mentioned in the introduction in which the water jet leaving the sanitary outlet fitting geometrically continues the contour of the water outlet or of the outlet water-conducting cross-sectional area of the water outlet and therefore assumes the cross-sectional geometry—perpendicular to the flow direction—of the inner contour of the sanitary outlet fitting, which inner contour differs from the visible outer contour only by the wall thickness of the outlet part. 
     This object is achieved according to the invention in the jet regulator of the type mentioned in the introduction in particular in that an aeration duct which is required for aerating the water jet is delimited, at a part of the periphery arranged so as to face away from the visible side of the outlet fitting, between the jet regulator outer periphery and the fitting inner periphery. 
     In the jet regulator according to the invention, the aeration duct required for aerating the water jet does not extend—as is conventional—over the entire periphery between the jet regulator periphery and the fitting inner periphery; said aeration duct is in fact delimited at a part of the periphery arranged so as to face away from the visible side of the outlet fitting. Since the air required for generating an aerated jet is inducted not at all sides but rather only at a part of the periphery arranged so as to face away from the visible side of the outlet fitting, the utilization of the available surfaces is increased, which is of particular technical and aesthetic significance in particular in the case of an outlet fitting which is wide but flat in terms of its clear fitting inner periphery. 
     To achieve the object stated above, a further, independently patentable proposal provides that the length of at least one dimension of the water jet, perpendicular to the flow direction thereof, at the outlet is equal to the outlet inner dimension minus two times the housing wall thickness of the jet regulator. 
     In order that the jet regulator according to the invention can satisfy even high demands and can be advantageously used in a wide variety of outlet fittings, a further independently patentable proposal provides that, to fasten or fix the jet regulator in the water outlet, at least one retaining element is provided which extends through a passage opening provided on the periphery of the fitting housing and which engages, with its end region protruding into the housing interior of the fitting housing, on the jet regulator so as to fix the latter. 
     The jet regulator according to the invention may, if required, be inserted into the water outlet of a sanitary outlet fitting from the direction of the outlet end side. To be able to detachably fasten and fix the jet regulator according to the invention there, at least one retaining element is provided which extends through a passage opening provided on the periphery of the fitting housing, with the retaining element engaging, with its end region protruding into the housing interior of the fitting housing, on the jet regulator so as to fix the latter. Since the at least one retaining element may engage on a jet regulator of any desired shape, the jet regulator according to the invention promotes design freedom in the design of new outlet fittings without it being necessary to accept losses in function. Since the jet regulator according to the invention does not require an external thread or a bayonet connection on its housing periphery, it is possible at least in visual terms for practically the entire clear cross section of the outlet fitting, with the exception of the jet regulator housing walls, to serve to conduct water. 
     The retaining element may duly also act on the jet regulator so as to fix the latter. However, in order that the comparatively thin-walled jet regulator is not deformed to an excessive extent, it may be advantageous for the retaining element to engage into a fastening opening on the jet regulator. 
     The retaining element may be designed as a retaining splint which extends though the passage opening on the fitting housing and through a fastening opening on the jet regulator. One preferred embodiment of the invention, however, provides that the retaining element is designed as a retaining screw which can be screwed into the passage opening and/or the fastening opening. 
     It is particularly advantageous if the retaining element has a self-tapping thread and if the retaining element, as it is screwed in, cuts a thread into the fastening opening. It is therefore not necessary for the thread flights in the passage opening on the one hand to be adapted to the thread flights in the fastening opening on the other hand. 
     The retaining element no longer has an objectionable appearance at the outer periphery of the fitting housing if the retaining element is designed as a set screw and/or if a tool engagement surface, which is preferably designed as an internal hexagon, is provided on the visible end region of the retaining element. 
     It is particularly expedient if the fastening opening has, on its inner periphery, a profile formed from axial projections and depressions. While the retaining element can cut into the axial projections with little force expenditure, the material which is displaced during the thread-cutting process can be pressed into the region of the depressions, where it no longer poses a problem. 
     To form a sparkling, soft water jet, it is advantageous if the jet regulator is designed as an aerated jet regulator and if an aeration duct, which is open toward the outlet end side of the water outlet, is provided at least in one partial region between the fitting inner periphery and the jet regulator. 
     Here, one preferred embodiment according to the invention provides that at least one fastening opening is arranged on the jet regulator housing in the region of the aeration duct. 
     Since no external thread is required on the jet regulator housing for inserting and fixing the jet regulator according to the invention, the jet regulator according to the invention may also have a non-circular and in particular an elongate and/or rectangular outer periphery. 
     A further independently patentable proposal according to the invention provides that the jet regulator mentioned in the introduction has a jet regulator housing with a peripheral push-in opening, that at least one push-in guide which is aligned transversely with respect to the jet regulator longitudinal axis is provided in the housing interior of the jet regulator housing, and that at least one preferably jet-forming insert part can be pushed into the at least one push-in guide from the direction of the push-in opening. In the jet regulator according to the invention, the insert parts additionally required for forming the water jet can be pushed into the housing interior of the jet regulator housing from the direction of a peripheral push-in opening. For this purpose, at least one push-in guide which is aligned transversely with respect to the jet regulator longitudinal axis is provided in the housing interior of the jet regulator housing, into which push-in guide at least one preferably jet-forming insert part can be pushed in laterally. 
     In order that the clear cross section of the water outlet can be utilized practically completely for forming the water jet, it is advantageous if at least one insert part which is provided in a push-in guide extends substantially over the clear passage cross section of the jet regulator housing. 
     One particularly simple and advantageous embodiment according to the invention provides that at least one insert part is of plate-shaped design. 
     To perform the function of a jet diffuser, of a homogenizing device and/or of a flow straightener, it may be advantageous if at least one insert part has a preferably jet-forming sieve, grate or mesh structure. 
     In order that the different functions can be optimized in the jet regulator according to the invention, it can be advantageous if at least two insert parts can be pushed into the jet regulator housing, which insert parts preferably have different sieve, grate or mesh structures. 
     In order to provide the jet regulator housing which has a lateral push-in opening with a sufficient level of stability, it may be advantageous if a jet diffuser device which is preferably designed as a perforated plate is integrally formed in the jet regulator housing. 
     To make it possible for the functional units which follow the jet diffuser to be pushed into the jet regulator housing, it is advantageous if the jet diffuser device is formed into the jet regulator housing upstream of the push-in opening at the inflow side. 
     To prevent undesired leakage currents between the jet regulator on the one hand and the fitting inner periphery on the other hand, it is expedient if an annular seal, a lip seal or similar encircling seal element is provided between the jet regulator and the fitting inner periphery. 
     A seal element of said type can be sealingly inserted in an effective manner between the jet regulator housing on the one hand and the fitting inner periphery on the other hand without it being necessary for the jet regulator housing to be deformed if the jet regulator housing supports, in the region of the jet diffuser device, a seal element which bears sealingly against the fitting periphery. 
     To prevent an undesired discharge of water from the push-in opening of the jet regulator housing and to prevent an uncontrolled induction of air through the push-in opening, it is advantageous if a seal is provided between the push-in opening and the fitting inner periphery. 
     Here, one embodiment according to the invention which is particularly simple and can be produced with little expenditure provides that the seal is designed, at least in regions, as a sealing lip or similar sealing projection which is integrally formed on the jet regulator housing. 
     Since the push-in opening can be sealed off at its inflow-side edge region even by means of the seal element which encircles around the jet regulator housing, it can be sufficient if the sealing lip or similar sealing projection extends linearly over the peripheral edge region, which delimits the push-in opening, of the jet regulator housing at both sides into the region of the seal element. 
     One embodiment of the invention provides that a cover is provided to close off the push-in opening and that at least one insert part supports at least one partial region of the cover. When the required insert parts are pushed into the push-in opening, then those partial regions of the cover which are provided on said insert parts finally close off the push-in opening completely. 
     To also close off the push-in opening in a satisfactorily sealing manner, it is expedient if at least one pressing projection is integrally formed on the cover or on at least one partial region of the cover, which pressing projection acts on the fitting inner periphery. The pressing projection which bears against the fitting inner periphery and which is pressed radially inward as the jet regulator is pushed into the fitting housing in turn presses the cover or cover partial region sealingly against the jet regulator housing. 
     To further promote the simple production of the jet regulator according to the invention, it is advantageous if the at least one pressing projection is integrally formed on the cover or on the partial region of the cover in the region of the insert part. 
     To prevent slipping or a release of the insert parts which have been pushed into the jet regulator housing, it may be advantageous if at least one insert part is secured or held in its push-in guide by means of a force-fitting action. 
     To prevent an undesired manipulation of the insert parts situated in the housing interior of the jet regulator housing, it may be advantageous if a sieve, grate or mesh structure is integrally formed into the jet regulator housing at the outlet side. The sieve, grate or mesh structure which is integrally formed on the jet regulator housing at the outlet side may not only have a jet-forming action but rather also simultaneously performs the function of a manipulation prevention device. 
     In order that it is possible for the jet regulator which has been pushed preferably completely into the fitting housing to be removed again from the fitting housing as required, it is expedient if at least one tool engagement point for a disassembly tool is provided on the jet regulator housing, preferably in the region of the aeration duct. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features of the invention can be gathered from the following description of the Figures in connection with the claims. The invention is described in yet more detail below on the basis of advantageous exemplary embodiments. In the Figures: 
         FIG. 1  shows the fitting housing, in a longitudinally cut-away view, of a sanitary outlet fitting which, in the region of its water outlet, has a jet regulator whose outline is elongate and rectangular so as to be matched in terms of shape, 
         FIG. 2  shows the fitting housing, having the jet regulator, from  FIG. 1 , 
         FIG. 3  shows the jet regulator, inserted into the water outlet of the fitting housing, from  FIGS. 1 and 2  in the region of a set screw which fixes the jet regulator, 
         FIG. 4  shows the fitting housing of the outlet fitting, illustrated in  FIGS. 1 to 3  in an end view of its water outlet, 
         FIG. 5  shows the jet regulator from  FIGS. 1 to 4  in an exploded perspective illustration, 
         FIG. 6  shows the jet regulator from  FIGS. 1 to 5  in a longitudinally sectioned perspective illustration, 
         FIG. 7  shows a detailed view of the jet regulator illustrated, in a longitudinally sectioned view, in  FIG. 6  in the region of the insert parts pushed into the jet regulator housing, 
         FIG. 8  shows the jet regulator illustrated in  FIGS. 1 to 7  in a cross-sectioned perspective illustration, with a disassembly tool engaging on the jet regulator, 
         FIG. 9  shows a cross section of the jet regulator from  FIGS. 1 to 8  fixed in the fitting housing, 
         FIG. 10  shows a further embodiment of a jet regulator inserted into a fitting housing of a sanitary outlet fitting, 
         FIG. 11  shows the jet regulator fixed in the fitting housing from  FIG. 10 , with the fitting housing being illustrated in the region of its water outlet which supports the jet regulator, 
         FIG. 12  shows the jet regulator, situated in the water outlet of the fitting housing, from  FIGS. 10 and 11  in the region of the set screw which fixes the jet regulator, 
         FIG. 13  shows the fitting housing of the outlet fitting illustrated in  FIGS. 10 to 12  in an end view of its water outlet, 
         FIG. 14  shows the jet regulator illustrated in  FIGS. 10 to 13  in an exploded perspective illustration, 
         FIG. 15  shows the jet regulator from  FIGS. 10 to 14  in a longitudinal section, 
         FIG. 16  shows a detailed view of the jet regulator illustrated in a longitudinally sectioned view in  FIG. 15 , 
         FIG. 17  shows the jet regulator from  FIGS. 10 to 16  in a cross-sectioned perspective illustration, with a disassembly tool engaging on the jet regulator outer periphery, 
         FIG. 18  shows a cross section of the jet regulator from  FIGS. 10 to 17  fixed in the water outlet of the fitting housing, 
         FIG. 19  shows the water outlet of a sanitary outlet fitting, wherein the housing of the sanitary outlet fitting is illustrated with hatching and the tube cross-sectional area which is bordered by said sanitary outlet fitting is illustrated without hatching, and 
         FIG. 20  shows a jet regulator similar to that in  FIGS. 1 to 18  in a plan view of its outflow-side end side, wherein the housing contour of the jet regulator and the flow straightener provided at the outflow side are illustrated with hatching and the water outlet areas bordered by said housing contour and flow straightener are illustrated without hatching. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1 to 9 and 10 to 18  illustrate different embodiments  1 ,  10  of a jet regulator which can be inserted into the water outlet  2  of a sanitary outlet fitting  3  from the direction of the outlet end side in order to form a homogenous, sparkling, soft and non-sputtering water jet there. To fasten and fix the jet regulator  1 ,  10 , which has a non-circular and in this case elongate rectangular outline, in each case one retaining element  4  is provided which extends through a passage opening  5  provided on the periphery of the fitting housing and which engages, with its end region protruding into the housing interior of the fitting housing, on the jet regulator  1 ,  10  so as to fix the latter. 
     For this purpose, the retaining element  4  engages into a fastening opening  6  which is provided laterally on the jet regulator housing  7 . The retaining element  4  is designed, in the exemplary embodiments shown in  FIGS. 1 to 9 and 10 to 18 , as a retaining screw and in particular as a set screw which can be screwed into the passage opening  5  on the fitting housing and into the fastening opening  6  on the jet regulator housing  7 . In order that the set screw  4  no longer has an objectionable appearance at the outer periphery of the fitting housing, the set screw  4  has, on its visible end region, a tool engagement surface, which is designed as an internal hexagon  8 , for a hexagonal key. 
     To facilitate the assembly of the jet regulator in the water outlet of the outlet fitting, the retaining element  4  has a self-tapping thread, by means of which the retaining element  4 , as it is screwed in, cuts a thread into the fastening opening  6 . As is clear from  FIGS. 5 and 6 or 14 , the fastening opening  6  has, at its inner periphery, a profile formed from axial projections and depressions, with the retaining element  4  cutting with its self-tapping thread into the projections of the fastening opening  6 , while the material which is thereby displaced can be pressed into the depressions. 
     The jet regulators  1 ,  10  illustrated in  FIGS. 1 to 9 and 10 to 18  are aerated jet regulators in which air is admixed to the water jet. To be able to induct the air required for the air admixture into the housing interior of the jet regulator  1 ,  10 , an aeration duct  9 , which is open toward the outlet end side of the water outlet  2 , is provided in one partial region between the fitting inner periphery and the jet regulator  1 ,  10 . The aeration duct  9  opens out in the region of aeration openings  11  which are provided in the jet regulator housing  7  and which lead, below a jet diffuser  12 , into the housing interior. Here, the fastening opening  6  is arranged on the jet regulator housing  7  in the region of the aeration duct  9 . 
     From a comparison of  FIGS. 5 to 7 and 14 to 16 , it is clear that the jet regulators  1 ,  10  shown here have a jet regulator housing  7  with a peripheral push-in opening  13 . Here, at least one push-in guide  14  which is aligned transversely with respect to the jet regulator longitudinal axis is provided in the housing interior of the jet regulator housing  7 , such that the insert parts  15  required for forming the water jet can be pushed into the push-in guide or push-in guides  14  from the direction of the push-in opening  13 . To be able to form the water jet over the entire cross section thereof, the plate-shaped insert parts  15  extend substantially over the entire clear passage cross section of the jet regulator housing  7 . It can be seen from  FIGS. 5, 9 and 14  that the insert parts  15  which serve here as a homogenization device have a jet-forming sieve or grate structure. 
     A jet diffuser device  12  which is designed as a perforated plate is integrally formed in the jet regulator housing  7  upstream of the push-in opening  13  at the inflow side. To prevent undesired leakage currents between the jet regulator housing  7  on the one hand and the fitting inner periphery on the other hand, an annular seal  16  is provided between the jet regulator  1 ,  10  and the fitting inner periphery. The annular seal  16  which is supported by the jet regulator housing  7  in the region of the jet diffuser device  12  can bear sealingly against the fitting housing without the risk of a deformation of the jet regulator housing  7  in said region, since the jet diffuser device  12  serves to stiffen the jet regulator housing  7  and counteracts an undesired deformation. 
     It can be seen from  FIGS. 1 to 10  that the push-in opening  13  of the jet regulator  1  can be closed off by means of a cover which is formed from a plurality of cover partial regions  17 ,  18  which are integrally formed on the insert parts  15 . Pressing projections  19  are integrally formed on said cover partial regions at the outside, which pressing projections  19  act on the fitting inner periphery. As the jet regulator  1  is inserted, said pressing projections  19  are clamped between the fitting inner periphery and the jet regulator housing  7  in such a way that the pressing projections  19  press the cover partial regions  17 ,  18  against the peripheral edge region, which delimits the push-in opening  13 , of the jet regulator housing  7  with a sufficient sealing action. 
     It can be seen from  FIGS. 10 to 18  that the push-in opening  13  of the jet regulator  10  is not assigned any cover. Instead, only a seal  31  is provided here between the push-in opening  13  and the fitting inner periphery, which seal  31  is intended to prevent an undesired discharge of water and an uncontrolled inflow of air. Said seal is also designed here at least in regions as a sealing lip  31  which is integrally formed on the outside of the jet regulator housing  7 . Since the push-in opening  13  is already sealed off at its inflow-side edge region by the annular seal  16 , the sealing lip  31  extends linearly over the peripheral edge region, which delimits the push-in opening  13  of the jet regulator  10 , of the jet regulator housing  7  at both sides into the region of the annular seal  16 . 
     It can be seen in  FIG. 7  that the insert parts  15  can be pushed with lateral guide rails  21  into the push-in guides  14  which are designed at both sides as guide grooves. Clamping strips  22  project from said guide rails  21 , which clamping strips  22  hold the insert parts  15  in their push-in guide  14  by means of a force-fitting action. 
     It is clear from a comparison of  FIGS. 8 and 9  that the insert part  15  which is situated directly downstream of the jet diffuser device  12  at the outflow side has at least one centering pin  23  which engages into a centering opening on the jet regulator housing  7 . Correct positioning between the jet diffuser device  12  on the one hand and the functional unit  15 , which follows said jet diffuser device  12  at the outflow side, on the other hand is ensured by means of the centering pin  23  engaging into the centering opening of the jet regulator housing  7 , in such a way that the individual jets formed in the jet diffuser device  12  impinge directly on a crossing node of the sieve or grate structure formed by the outflow-side insert part  15 . 
     A sieve or grate structure  24  is integrally formed on the jet regulator housing  7  of the jet regulator  1 ,  10  at the outlet side, which sieve or grate structure firstly serves as a flow straightener and secondly also constitutes a manipulation prevention device which is intended to prevent unauthorized manipulation of the insert parts  15  situated in the housing interior of the jet regulator housing  7 . 
     It is clear in particular from  FIGS. 8 and 17  that a tool engagement point  25  which is designed as a hook-in projection is provided on the jet regulator housing  7  preferably in the region of the aeration duct, for a disassembly tool  26  which is designed here in the manner of a hook. The jet regulator housing  7  which is situated in the water outlet  2  of the sanitary outlet fitting  3  can be removed from the water outlet as required by means of the disassembly tool  26  which engages behind the tool engagement point  25 . 
     It is a particular advantage of the jet regulators  1 ,  10  illustrated here that their outlet area A 4  which is calculated without the jet regulator housing or the sieve or grate structure and which is bordered and unhatched in  FIG. 20  is kept comparatively large in relation to the clear inner cross-sectional area A 3 , which is bordered and unhatched in  FIG. 19 , of the water outlet  2 . Calculating the area ratio of the sum A 4  of the individual water outlet areas (illustrated in  FIG. 20 ) out of the structure of the jet regulator in relation to the unhatched structural area in the sanitary outlet fitting shown in  FIG. 19  yields a water outlet area A 4  of 50% of the available gross area A 3  of the outlet bore at the water outlet  2  of the outlet fitting  3 . Here, it may be advantageous if said water outlet area is greater than/equal to 0.3 and in particular greater than/equal to 0.4 of the gross area, preferably 0.45 and in particular greater than/equal to 0.5 of the gross area of the outlet bore of the fitting outlet. Viewed from the visible side S, the length L 2  of the water jet at the outlet of the jet regulator in comparison to the length L 1  of the clear cross-sectional length of the outlet fitting is &gt;0.8, preferably 0.9. 
     From a comparison of  FIGS. 1 to 9  on the one hand and  FIGS. 10 to 18  on the other hand, it is clear that the jet regulator  1  has, for each of its insert parts  15 , in each case one push-in guide  13 , whereas the jet regulator  10  has only one push-in guide  13 , into which a plurality of insert parts  15  can be pushed. The insert parts  15  of the jet regulator  10  shown in  FIGS. 10 to 18  are held with a spacing to one another by means of spacer frames  35 . 
     It may be advantageous for the water flow to be subjected to a greater resistance at the side edges of the insert parts  15 . For this purpose, it is possible for a greater number of webs which form the sieve or grate structure to be provided at the side edges of the insert parts  15 , such that only a relatively small water quantity can pass and spraying of the emerging water jet at the jet periphery can be prevented or at least reduced. If, instead, a lower number of webs are provided at the side edges of the insert parts  15 , and if the water jet is thereby subjected to a reduced resistance in the region of the side edges of the insert parts, it is possible to ensure the cylindrical shape of the emerging water jet over a relatively long distance, which is desirable in particular in the case of elongate jet regulators. 
     It is particularly advantageous if the sieve or grate structures of the insert parts  15  which are positioned in series with one another to be aligned with the gaps of the adjacent structures. Even if the insert parts  15  are of identical design, this is possible by means of a lateral offset of the sieve or grate structures for example by approximately half of a mesh width. Instead, it is also possible to use asymmetrical sieve or grate structures which can be aligned with the gaps of the adjacent structures by means of a simple rotation of the identically-designed insert parts  15 . 
     In each case one ancillary sieve  30  is positioned upstream of the jet regulators  1 ,  10  at the inflow side, which ancillary sieve  30  filters out the dirt particles contained in the water.

Summary:
A jet regulator ( 1 ) which can be installed into the water outlet ( 2 ) of a sanitary outlet fitting ( 3 ), on the front side thereof. The jet regulator ( 1 ) can be arranged in a detachable or fixed manner in the water outlet ( 2 ). The jet regulator includes at least one holding element ( 4 ) for arranging or fixing the jet regulator ( 1 ) in the water outlet ( 2 ), with the holding element being inserted into a passage opening ( 5 ) on the perimeter of the fitting housing and engaging, in a fixed manner, with the end area thereof, the jet regulator ( 1 ), which projects into the interior of the fitting housing. The jet regulator ( 1 ) has a jet regulator housing ( 7 ) with an insertion opening ( 13 ) on the peripheral side, with the housing including in the interior at least one insertion guide ( 14 ) which is oriented across the jet regulator housing longitudinal axis, and at least one, preferably jet-producing, converter piece ( 15 ) can be inserted into the at least one insertion guide ( 14 ) from the insertion opening ( 13 ).