referring now to fig1 the surgical instrument illustrated therein is for clearing obstructions in coronary arteries . the instrument comprises a body 10 formed in two parts consisting of a hollow bushing 12 and sleeve 14 extending forwardly of the bushing 12 . the sleeve 14 has an open forward end 16 and a rear end which has been inwardly deformed so as to define a stop 18 . the stop 18 abuts against a step 20 provided on the external surface of the bushing 12 between a larger diameter forward end portion 22 and a smaller diameter rear end portion 24 thereof . the sleeve 14 is slidable rearwardly ( i . e . to the left as viewed in the drawing ) relative to the bushing 12 from an extended position in which ( a ) the stop 18 abuts against step 20 and ( b ) a pip 26 formed by localized inward deformation of the sleeve 14 engages in an annular groove 28 formed in the surface of the larger diameter forward end portion 22 of the bushing 12 . the engagement of the pip 26 in the groove 28 serves as a releasable detect mechanism for retaining the sleeve 14 in its extended condition as illustrated in the drawing . the surgical instrument further includes a cutter member 30 which is mounted in the body 10 via a stepped root portion 32 . the root portion 32 engages in a stepped bore of the bushing 12 and is retained therein by an inwardly upset annular lip 34 . the root portion 32 is rotable relative to the bushing 12 which acts as a bearing therefor . the root portion 32 carries a tapered shaft 36 which projects forwardly and axially of the sleeve 14 so as to pass through the open end 16 of the sleeve 14 . at a position which is spaced forwardly of the bushing 12 , the shaft 36 provided with integral screw threading 38 thereon . the screw threading 38 starts within the sleeve 14 and extends forwardly to the tip of the shaft 36 , the screw threading tapering inwardly over a region thereof which is disposed externally of the sleeve 14 . the forward end of the screw threading 38 and the forward end of the shaft 36 are relatively blunt and have no sharp edges whereby to minimize risk of inadvertent penetration of the arterial wall during use . the portion of the screw threading 38 which is disposed within the sleeve 14 has a peripheral surface which is relatively sharp and which is in close sliding contact with the internal wall surface of the sleeve 14 . the root portion 32 has a blind axial bore 40 therein in which one end of a flexible drive cable 42 is firmly secured . the drive cable 42 extends through a flexible outer sleeve or sheath 44 and is rotatable relative thereto . in the drawing , only a short length of the cable 42 and sheath 44 are illustrated . in practice , these latter two parts will extend for a considerable distance rearwardly of the body 10 , with the cable 42 terminating externally of the sheath 44 in a formation which facilitates manual rotation of the cable 42 at a remote location . in use , the body 10 including the cutter member 30 is passed along an artery of a conscious patient until it meets the obstruction to be cleared . this is effected by appropriately maneuvering the body 10 using the sheath 44 with the aid of an x - ray camera when the body 10 has been maneuvered into the correct position along the artery , the cable 42 is rotated relative to the sheath 44 so as to cause the cutter member 30 to rotate in a direction in which it causes the forward end of the screw threading 38 to dig into the material of the obstruction and thereby draw itself and the body 10 along the artery and further into the obstruction . a circular cutting action is provided by mutual cooperation of ( a ) the outer cutting edge of that portion of the screw threading 36 which is disposed within the sleeve 14 and ( b ) the inner peripheral wall of the sleeve 14 . the cut material is carried between the flights of the screw threading 38 within the sleeve 14 to be discharged into collection chamber 46 disposed within the sleeve 14 and between the screw threading 38 and the bushing 12 . when it is seen on the x - ray camera that the obstruction has been cleared , rotation of the cable 42 is stopped and the body 10 is withdrawn by employing a pulling action on the sheath 44 . once the body 10 has been removed from the artery 10 , material which has been collected in the chamber 46 can be easily removed upon retraction of the sleeve 14 relative to the bushing 12 . it will be appreciated that , because the cutting action takes place by cooperation of the screw threading 38 with the internal wall of the sleeve 14 , there is a minimum of risk that the wall of the artery will be inadvertently penetrated . in order to assist insertion of the body 10 and to minimize risk of damage to the artery wall , the leading edge of the sleeve 14 around the open end 16 is conveniently radiussed . in order to facilitate withdrawal of the body 10 along the artery , the rear end of the smaller diameter portion 24 of the bushing 12 and the rear edge of the sleeve 14 adjacent the stop 18 are both appropriately tapered . referring now to fig2 the surgical instrument illustrated therein is similar to that of fig1 and similar parts are accorded the same reference numerals but in the 100 series . in this example , cutter member 130 is rounded at its forward end to minimize risk of arterial wall penetration in use . sleeve 114 of body 110 is flexible , but is fitted with a metal ring 150 to define open forward end 116 . the rotary cutting action takes place between cutter member 130 and metal ring 150 . the body is internally screw - threaded at its rearward end to engage with external screw - threading on the forward end of sheath 144 . the screw - threading is directed so that the screw - threaded joint between the body 110 and the sheath 144 tends to be tightened during forward rotation of the cutter member 130 in use . the sheath 144 may have a smaller diameter than the body 110 . at its rear end , the sheath 144 is provided with a formation 152 to facilitate gripping of the instrument during use . cable 142 passes out of the rear end of the sheath 144 and is connected to a hand wheel 154 ( only schematically shown ). the cable 142 is connected to shaft 136 by connecter 156 which also acts as a bearing . referring now to fig3 and 4 , the surgical instrument illustrated therein is similar to that of fig2 and similar parts are accorded the same reference numerals but in the 200 series . in this example , cutter member 230 and shaft 236 have aligned axial passages therethrough , and the cable 142 is replaced by a flexible tube 242 which is joined to the shaft 236 at connector 256 which is hollow . these axial passages typically have a diameter of about 0 . 4 - 0 . 46 mm . in use , a guide wire 258 typically having a diameter of about 0 . 36 mm is passed into and along the artery to a position beyond the location at which the obstruction exists . the body 210 can then be passed along the artery to the location of the obstruction whilst being continually guided by the wire 258 so as to minimize the risk of penetration of or damage to the wall of the artery . referring now to fig5 of the drawings , the surgical instrument illustrated therein is similar to the preceding examples and similar parts are accorded the same reference numerals but in the 300 series . in this example , sheath 344 has a diameter which is substantially less than that of body 310 except for an enlarged forward end portion 360 by which it is screw - engaged with body 310 . a bushing 356 around cable 342 assists in correctly locating the latter within the portion 360 . a washer 362 in the body 310 and lodged between the portion 360 and an internal rib 364 serves to limit passage of debris into the sheath 344 from chamber 346 and can be readily removed upon detachment of the body 310 from the portion 360 to facilitate removal of debris from the chamber 346 after use .