a modified form of instrument support structure , according to the present invention , is shown at 400 in fig8 and 9 . the instrument support structure 400 consists of a base 402 , a connector 404 and a sleeve 406 shown in an operative position for the instrument support structure in fig8 . the base 402 has a flat surface 408 to facially engage a surface 410 on a tissue 412 through which an incision 414 has been made . the base has a body 416 defining a receptacle 418 for the connector 404 . the receptacle 418 is bounded by a concave surface 420 that is slightly larger than , but matched in shape to , the spherical outer surface 422 of the connector 404 . with the connector 404 operatively positioned within the receptacle 418 , the connector 404 pivots universally against and relative to the base 402 . the connector 404 has a through bore 424 which accepts the sleeve 406 , which is preferably cylindrical , and slidable lengthwise within the through bore 424 . the connector 404 has a cylindrical neck 426 which closely surrounds the sleeve outer surface 428 . the neck 426 has a bore 430 therein to accept a set screw 432 , which can be tightened to fix the relative positions of the connector 404 and sleeve 406 . the sleeve 406 thus follows the universal movement of the connector 404 relative to the base 402 . this movement is accommodated by providing an enlarged bore 434 communicating from the receptacle 418 through the base 402 , whereby the subassembly consisting of the connector 404 and sleeve 406 can pivot , as in the direction of double - headed arrow 436 , within a range limited by an annular shoulder 436 surrounding the bore 434 . the relative positions of the base 402 and connector 404 can be fixed by a set screw 438 which is directed through an upper portion 440 of the base 402 and against the outer surface 422 of the connector 404 above the center point 442 so that the advancing set screw 438 cams the connector 404 progressively into the receptacle 418 . the upper portion 440 of the base 402 is made relatively thin , as from plastic , to be deformable sufficiently to allow the connector 404 to be snap - fit to the base 402 . as the connector 404 is driven in the direction of the arrow 444 in fig9 the restricted entryway 446 to the receptacle 418 is progressively enlarged by the connector outer surface 422 until the connector 404 fully seats in the receptacle , whereupon the base potion 440 springs back to its undeformed state , shown in fig8 . camming open of the entryway 446 is facilitated by having an annular , tapered edge 448 at the entryway 446 to the receptacle 418 . with this structure , the base 402 and connector 404 can be conveniently assembled on site by the surgeon . the invention contemplates several different ways of setting up the instrument support structure 400 . in one such method , the connector 404 is preassembled to the base 402 , as shown in fig9 . the subassembly consisting of the base 402 and connector 404 is then placed against the tissue 412 so that the incision 414 coincides with the opening defined by the base bore 434 . the sleeve 406 can then be directed through the assembled base 402 and connector 404 . the sleeve 406 has an inflatable bladder / membrane 450 at the distal end 452 thereof . the bladder / membrane 450 is selectively placed in an inflated state , as shown in solid lines in fig8 and a collapsed state , as shown in phantom lines in fig8 . in this latter state , the bladder / membrane 450 does not project significantly outwardly from the peripheral sleeve surface 428 . the bladder / membrane 450 is placed selectively in its expanded and co / lapsed state by an inflating / deflating means , shown schematically at 453 in fig8 . the bladder / membrane 450 is initially deflated to allow the sleeve 406 to be directed through the connector 404 without interference between the bladder / membrane and the sleeve 406 . the sleeve 406 is extended sufficiently through the connector 404 that a distal portion 454 thereon is fully exposed within a cavity 456 surrounded by the tissue 412 . by then inflating the bladder / membrane 450 through the means 453 , an annular shoulder 458 is defined by the bladder / membrane 450 . the sleeve 406 can then be dram upwardly to bear the shoulder 458 against the inside surface 460 of the tissue 412 . the subassembly , consisting of the base 402 and connector 404 , is then slid downwardly towards the distal sleeve end 452 until the tissue 412 is firmly captured between the flat base surface 408 and the shoulder 458 on the bladder / membrane 450 . by then tightening the set screw 432 , the tissue 412 will remain positively captive between the base 402 and membrane / bladder 450 . according to the invention , it is preferred that the bottom edge 462 of the connector 404 be adjacent to the flat base surface 408 with the base 402 and connector 404 in operative relationship . the bottom edge 462 can be slightly above the surface 408 , slightly therebelow , or flush therewith . if the base 402 were shortened in a vertical direction , as indicated by the phantom lines 464 , the connector 404 would project slightly below the base surface 408 and would thus be urged directly against the tissue 412 as the support structure 400 is assembled to the tissue 412 . this enhances both the stability of the support structure 400 and the integrity of the seal around the incision 414 . with the tissue 412 captive between the base 402 and bladder / membrane 450 , the sleeve 406 can be tilted about its length by pivoting the connector 404 pivotably against the base surface 420 . once a desired relative position between the connector 404 and base 402 is established , the set screw 438 can be tightened . with the support structure assembled , an instrument 465 can be directed through a seal 466 , at the proximal end 468 of the sleeve 406 , and the sleeve 406 to thereby enter the cavity 456 . the sealing structure 466 could take any form known to those skilled in the art . similarly , the instrument 465 can take any of a number of different forms . as shown in the drawings , the instrument 465 is a forceps having jaws 470 , 472 which are movable selectively towards and away from each other through the operation of separate control handles 474 , 476 , which remain at all times exposed at the proximal end 468 of the sleeve 406 . a clamp 478 , of known construction can be used to fix the position of the instrument 465 relative to the sleeve 406 . when it is desired to change the orientation or the extension of the sleeve 406 within the cavity 456 , the clamp 478 , and set screws 432 , 438 can be selectively released and reset after a desired new position is selected . the invention contemplates other methods of setting up the instrument support structure 400 . for example , the base 402 can be initially placed against a tissue 412 , after which the connector 404 with the sleeve 406 already therein , can be snapped into the base 402 as the sleeve 406 is advanced through the tissue 412 . still further , the base 402 , connector 404 and sleeve 406 can all be preassembled before the sleeve 406 is directed through the tissue 4 12 . regardless of the manner of assembly , the connector 404 firmly holds the connector 402 against the tissue 412 at a point adjacent thereto for maximum stability . the stability of the support structure 400 is further enhanced by configuring the base 402 so that the flat surface 408 thereon makes contact with the tissue 412 over a substantial area . in one form , the flat base surface 408 has a rectangular shape . it of course could be square , as indicated with dotted lines 480 . a suitable dimension for the rectangular surface 408 in fig1 has a width ( w ) of 6 cm and a length ( l ) of 9 cm . the length l for the plate 408 is preferably aligned to be parallel with the primary path of pivoting for the sleeve 406 . this gives maximum stability to the base 402 . as can be seen , with the rectangular base in place and the length of the base 402 aligned between the ends a and b in fig8 any tendency to lift the end a is resisted by the end b digging into the tissue 412 . similarly , any tendency to lift end b is resisted by end a which digs into the tissue 412 . consequently , a substantial potion of the surface 408 remains in contact with the tissue 412 , even as the sleeve 406 is moved and the base 402 tips in use . fig1 shows an alternative , circular shape for the base surface 408 . regardless of the shape of the base 402 , it is desired that the effective diameter ( i . e . the distance from the center to the closest edge thereof ) be on the order of 2 - 9 cm . the invention further contemplates various different structures to join a connector and base . in fig1 a support structure 400 &# 39 ; is shown including a connector 404 &# 39 ; and a base 402 &# 39 ;. the base 402 &# 39 ; has one , and preferably a plurality , of tabs 482 which are each pivotable about a pin 484 between a holding position , shown in solid lines in fig1 , and a release position , wherein the tab 482 is pivoted in the direction of arrow 484 away from the connector 404 to allow the connector 404 to be separated from the base receptacle 418 &# 39 ;. a ratchet mechanism 486 can be provided to releasably maintain the tabs 482 in their holding position . the tabs 482 can be pivoted to a release position to allow introduction of the connector 404 &# 39 ;, after which the tabs 482 can be ratcheted into their holding position by a simple pressing step . the support structure 400 &# 39 ; also shows a layer 488 , which has an adhesive on both sides 490 , 492 , to bond to the base surface 408 &# 39 ; and the tissue 412 . this gives even greater stability to the support structure 400 &# 39 ; on the tissue 412 . fig1 shows a modified form of support structure 400 &# 34 ;, wherein the base 402 &# 34 ; defines a u - shaped receptacle 418 &# 34 ;, through which a connector 404 &# 34 ; can pass , without interference , into the operative position shown in fig1 . diametrically opposite tabs 494 , 496 can be advanced by set screws 498 , 500 at a location above the center 502 of the connector 404 &# 34 ; to releasably maintain the connector 404 &# 34 ; firmly in the receptacle 418 &# 34 ;. in fig1 , a modified form of support structure is shown at 400 &# 39 ;&# 34 ;, to include a base 402 &# 39 ;&# 34 ; and a connector 404 &# 39 ;&# 34 ;. a cap 504 is threaded into the base 402 &# 39 ;&# 34 ; to captively hold the connector 404 &# 39 ;&# 34 ; in its operative position on the base 402 &# 39 ;&# 34 ;. by threading the cap 504 downwardly , the connector 404 &# 39 ;&# 34 ; is pressed positively against the receptacle surface 420 &# 39 ;&# 34 ; so that it is thereby fixed . an optional set screw 506 can be provided to more positively fix the relative positions of the connector 404 &# 39 ;&# 34 ; on the base 402 &# 34 ;&# 39 ;. a further modified form of support structure is shown in fig1 at 400 &# 34 ;&# 34 ;. a base 402 &# 34 ;&# 34 ; has a downwardly opening , u - shaped receptacle 418 &# 34 ;&# 34 ; which allows the connector 404 &# 34 ;&# 34 ; to be directed upwardly into its operative position against a surface 420 &# 34 ;&# 34 ; defined by the base 402 &# 34 ;&# 34 ;. once the connector 404 &# 34 ;&# 34 ; is seated in its operative position , a retaining plate 508 can be slid radially through a slot 510 in the base 402 &# 34 ;&# 34 ;. the plate 508 has a through opening 512 to accommodate an instrument extending through the connector 404 &# 34 ;&# 34 ;. the through opening 512 is bounded by an edge 514 which vertically supports the connector 404 &# 34 ;&# 34 ; and guides the same as it pivots , in conjunction with the base surface 420 &# 34 ;&# 34 ;. it is preferred that the diameter of the connectors 404 , 404 &# 39 ;, 404 &# 34 ;, 404 &# 39 ;&# 34 ;, and 404 &# 34 ;&# 34 ; be at least two times the diameter of the sleeve 406 . the foregoing disclosure of specific embodiments is intended to be illustrative of the broad concepts comprehended by the invention .