a surgical trocar is illustrated in fig1 and designated generally by the reference numeral 10 . the trocar 10 is a narrow elongate instrument having a distal end 12 and a proximal end 14 . it is typically configured along a longitudinal axis 16 and is generally circular in radial cross - section . it is the purpose of the trocar 10 to provide a channel through a tissue barrier in order to provide access across the barrier into a body cavity . by way of example , an abdominal wall 18 is illustrated in fig1 and typically includes a layer of skin 21 , a layer of fat 23 , and a layer of muscle 25 which form the tissue barrier to an abdominal cavity 27 . the trocar 10 typically includes an elongate tube or cannula 32 having a cylindrical configuration and a wall thickness such as 0 . 015 inches . the cannula 32 has an interior bore or channel which is typically in a range of diameters between 5 to 12 millimeters . it is the purpose of the trocar 10 to pierce , cut , incise or otherwise puncture the tissue barrier , such as the abdominal wall 18 , and to leave the cannula 32 extending through that incision with the channel 34 providing an access port into the cavity 27 . through this channel 34 , various surgical instruments such as cutters , clamps , traction devices , visualization devices , aspirators and irrigators can be positioned and manipulated to perform a particular surgical procedure within the cavity 27 . the trocar 10 also includes an obturator which extends through the cannula 32 and provides the means for cutting through the wall 18 to provide for insertion of the cannula 32 . in the past , obturators have been formed from solid metal rods which have been sharpened to a point at the distal end 12 of the trocar 10 . the forces necessary to puncture the abdominal wall 18 with such a device have been considerable due primarily to the presence of the muscle layer 25 in the wall 18 . puncturing of the wall 18 with such devices has been further complicated with the close proximity of important organs such as the liner 38 which , in some patients may actually be attached to the abdominal wall 18 by an adhesion 41 . in order to avoid puncturing these organs , it has been an absolute requirement that the forward movement of the trocar 10 be stopped as soon as the distal tip of the obturator pierces through the interlayer of the wall 18 . these conflicting requirements to provide a significant puncture force and then to immediately halt application of that force , have made the trocars of the past both difficult and dangerous to use . in accordance with the present invention an obturator 36 includes a distal tip 43 which extends beyond the distal end of the cannula 32 . this tip 43 is provided with at least one electrocautery wire 42 which can be activated through a conductor 45 by a radiofrequency generator 47 . initially the trocar 10 is advanced until the wire 42 and the tip 43 are brought into contact with the tissue barrier such as the wall 18 . activating the wire 42 with radiofrequency energy causes the contacted cells to vaporize forming an opening or incision in the wall 18 . with the application of a relatively minimal force , the trocar 10 can be advanced through the tip 45 clears the inner layer of the wall 18 , such as the muscle layer 25 . at this point , it is desirable to de - energize the cutting element or wire 42 so that any further forward movement of the trocar 10 does not accidently cut an interior organ such as the liner 38 . various apparatus and methods for sensing this particular location and inhibiting further cutting is discussed in greater detail below . at the proximal end 14 of the trocar 10 the cannula 32 is attached to a valving mechanism 50 which can be of the type disclosed by moll in u . s . pat . no . 4 , 654 , 030 , or disclosed by applicant in copending u . s . patent application ser . no . 07 / 630 , 078 filed on dec . 19 , 1990 . the obturator 36 includes an elongate shaft 52 which may include interior portions defining an axial channel 53 for the conductor 45 . this shaft 52 extends through the valving mechanism 50 as well as the cannula 32 with the tip 43 extending beyond the distal end of the cannula 32 . a finger knob 54 can be attached to the shaft 52 of the obturator 36 to facilitate application of the minimal axial force required to advance the trocar 10 . upon penetration of the wall 18 , this finger knob 54 can be withdrawn proximally through the cannula 32 and the valving mechanism 50 . in this manner , the cannula 32 is left in place with the interior channel 34 providing access across the abdominal wall 18 into the body cavity 27 . in the illustrated embodiment , the distal tip 43 of the obturator 36 has the configuration of a duck bill . it is defined primarily by a pair of opposing ramps 56 , 58 which extend from the outer surface of the shaft 52 inwardly with progressive distal positions along the ramps 56 , 68 . at the distal end of the obturator 36 , the ramps 56 and 58 terminate in a pair of lips 61 , 63 respectively which define an interior recess 65 that is configured to receive the wire 42 . it will be noted that the lips 61 , 63 extend slightly distally of the wire 42 by a particular distance . as the trocar 10 is moved forwardly , these lips 61 , 63 are the only part of the trocar 10 which actually touch the wall 18 . this particular distance is carefully selected , however , so that when the lips 61 , 63 touch the wall 18 , the wire 42 is close enough to the wall 18 that the most proximate cells vaporize to create the desired incision . this cutting by close proximity is commonly referred to as arcing . as used herein the cutting element , such as the wire 42 , is deemed to be in contact with the wall 18 if the desired arcing or cutting occurs . in the embodiment of fig1 the electrocautery technique is monopolar ; that is , only a single pole , such as the positive pole , is carried by the trocar 10 . in this type of technique , the patient is laid directly on a large plate which provides the second pole required by the electrocautery system . the rf generator 47 produces a radio frequency electrical energy signal which travels through the positive electrode connected to the wire 42 and through the body of the patient , to the negative pole at the plate 67 . where this conduction path is large in cross - section , the current density is very small . however , in proximity to the wire 42 the current path is very small in cross - section so the current density is quite large . it is this large current density which results in vaporizing the cells of the wall 18 in proximity to the wire 42 . it will be apparent that a bipolar electrocautery technique is equally applicable to this invention and may actually be preferred in some circumstances . a bipolar embodiment is illustrated in fig3 and 4 wherein the shaft 52 of the obturator 36 is separated axially into two half - shafts 72 and 74 each having an axial cross - section of a half circle and each including one of the duck bills associated with the nose 43 . each of the half - shafts 72 , 74 has an inner surface which defines a recess near the associated tip 61 , 63 respectively . for example , the half - shaft 72 includes an inner surface 76 which defines a recess 81 near the lip 61 . similarly , the half - shaft 74 has an inner surface 78 which defines a recess 83 near the lip 63 . these recess 81 and 83 are configured to receive a pair of blades 85 , 87 respectively which are connected to the two electrical poles of the rf generator 47 . thus , the blade 85 is connected through a conductor 90 to the positive pole of the generator 47 while the blade 87 is connected through a conductor 92 to the negative pole of the generator 47 . a layer of insulation 94 is sandwiched between the surfaces 76 , 78 to separate the blades 91 , 93 . in this bipolar embodiment , current travels from the blade 91 through the tissue wall 18 , around the insulator 94 and into the blade 93 . the exact configuration of the cutting elements 42 , 91 and 93 in these embodiment is not important as long as the desired current density can be maintained . thus the wire 42 and the blades 91 and 93 may be interchangable in the fig1 and fig2 embodiments . in a further embodiment of the invention illustrated in fig5 and 6 , the shaft of the obturator 36 is again separated into the half - shafts 72 and 74 . along either or both of the interior surfaces 76 , 78 a conducting material can be doped into or otherwise deposited on the material forming the half - shaft 72 , 74 . for example , in a preferred embodiment , the half - shafts 72 , 74 are formed from glass or sintered ceramic and the layers 96 , 98 contain a conductive polymer or metal which is doped into the surfaces 76 , 78 respectively . in a monopolar embodiment , only one of the layers 96 , 98 is required . in the bipolar embodiment illustrated both layers 96 , 98 are required as well as the insulation layer 94 which separates the two half - shafts 72 , 74 . in this particular embodiment , the doped layer , for example the layer 98 , can form both the cutting element , such as the blade 93 , as well as the associated conductor , such as the conductor 92 . the layer 98 may also include a region of epitaxial layering which forms a logic circuit 101 discussed in greater detail below . other embodiments of the invention which take advantage of the electrocautery technique are illustrated in fig7 through 11 . in the fig7 embodiment , which is also shown in the end view of fig8 the distal end of the obturator 36 is formed with three planes or lands 112 , 114 and 116 which extend from the outer surface of the shaft 52 distally to a point 118 . in this embodiment , a wire 121 is disposed along each of the lines defined by the intersection of the lands 112 , 114 and 116 . in operation , the wires 121 cut the tissue of the wall 18 along three lines so that the incision is defined by three flaps of the tissue . this particular embodiment may be desirable where it is necessary to equalize pressures of the tissue on the outer surface of the cannula 32 . in the embodiment of fig9 the distal end of the obturator 36 is formed with a projection 123 which extends distally and axially of the trocar 10 . a recess or channel 125 is defined to extend generally radially across the projection 123 . the cutting element or wire 142 in this embodiment passes down the axial channel 53 in the obturator 36 and passes outwardly of the shaft 52 through a port 127 at the distal end 12 . the wire 42 then passes through the recess 125 in the projection 123 and back into the shaft 52 through a port 130 . this embodiment is of particular interest since either end of the wire 42 can be advanced distally within the axial channel 53 to expand the width of the wire 42 at the distal end of the trocar 10 . in fact , both ends of the wire 42 can be moved distally to extend the exterior portions of the wire 42 , for example to the dotted position 132 illustrated in fig6 . this position 132 has a significantly greater width and therefore provides a wider incision in the wall 18 . thus the wire 42 can be advanced and retracted within the axial channel 53 to vary the size and shape of the resulting incision . in the embodiment of fig1 , 11 , the cutting element has the configuration of a blade 134 similar to that first discussed with reference to fig3 . in this case , the blade 134 can be advanced beyond the distal end of the shaft 52 associated with the obturator 36 . it can also be retracted into a recess 136 in the shaft 52 in order to inhibit any further cutting of the tissue associated with the wall 18 . the invention is not limited to the electrocautery embodiments or techniques disclosed above . rather , other types of cutting elements can be disposed at the distal end of the obturator 36 to increase the cutting capabilities of the trocar 10 . one such embodiment is illustrated in fig1 which provides for optical cutting in combination with a laser 105 . the optical energy provided by the laser 105 is conducted through an optical fiber 107 to a cutting element or lens 109 which concentrates the energy to form the required incision in the wall 18 . it will be apparent to those skilled in the art that ultrasonic cutting is equally applicable to the present invention . in such a device , the conductor would transmit energy not in the radiofrequency range but rather in the ultrasonic range between 100 khz and 1 . 2 mhz . as this energy emanates from the cutting element at the distal end of the trocar 10 , the energy can be focused to activate the proximate cells and cause those cells to vaporize . thus the ultrasonic cutter would function much as a microwave . one advantage of this system is that ultrasonic cutting does not require the two electrical poles associated with both monopolar and bipolar radiofrequency cutting . focusing of the ultrasonic energy could be achieved with suitable wave guides . one of the most significant problems confronting trocar procedures of the past has been associated with the need to compromise two conflicting requirements : 1 ) the requirement to provide a significant axial pressure to force the trocar 10 through the wall 18 ; and 2 ) the need to immediately cease application of that significant force upon penetration of the wall 18 . attempts have been made to address this problem automatically and mechanically with the provision of a protective sheath armed in a rearward position but bias to spring to a distal position covering the distal tip of the obturator 36 upon penetration of the wall 18 . this attempt to avoid relying totally on the surgeon for both of the conflicting requirements has met with only limited success . since the protective sheath has necessarily been larger than the diameter of the obturator 36 , the distal end 12 if the trocar has been required to move beyond the point of penetration in order to clear the distal end of the shield . in an attempt to provide reduced insertion forces , the angle between the axis 16 and the lands 112 , 114 , and 116 has been reduced . while this has decreased the angle of inclination associated with these lands 112 , 114 , and 116 , it is also extended the length of the lands 112 , 114 , and 116 rearwardly along the shaft 52 . this is necessarily required that the protective sheath be thrown a greater distance in order to cover the point 118 of the obturator 36 . the critical timing of this sheath response has not been sufficient to avoid the dramatic consequences associated with interior cutting . building on the advantages associated with the present invention whereby cutting by the obturator 36 is accomplished with electrical or optical energy , means can now be provided to sense complete penetration of the wall 18 and to cease further cutting by the cutting element . with reference to fig1 , a particular embodiment of the obturator 36 may include means disposed near the distal end 12 of the trocar 10 for sensing penetration of the wall 18 by the obturator 36 . the sensor 141 will typically be connected to a penetration detection and response circuit 143 which controls the rf generator through a pair of conductors 158 , 161 . in the illustrated embodiment , the current 143 includes an energy source 145 for energizing the sensor 141 through one or more conductors 147 . upon penetration of the wall 18 , the sensor 141 provides signal characteristics on a conductor 149 . in a particular embodiment , a switch 152 can be included in the response circuit 143 and provided with characteristics responsive to the signal on the conductor 149 to deactivate the generator 47 . thus the switch 52 has characteristics for closing when the sensor 141 is in proximity to the tissue and for automatically opening when the sensor 141 detects penetration of the wall 18 . this switch 152 can be interposed in series with a foot pedal 154 which includes a manual switch 156 for providing continuity between the two conductors 158 , 161 which activate the generator 47 . under certain circumstances , it may be possible that the sensor 141 would detect the absence of pressure if the physician merely discontinues forward movement of the trocar 10 through the wall 18 . under these circumstances , a physician might decide to proceed with further cutting and therefore desire that the rf generator 47 be reactivated . this can be easily accomplished by a circuit 143 which is responsive to further pressure on the sensor 141 to close the switch 152 . however , in a particular embodiment , it may be desirable to provide lock - out characteristics which require some manual switching by the physician in order to reactivate the generator 47 . such a circuit is illustrated schematically in fig9 wherein the switches 152 and 156 are both responsive to the signal characteristics on conductor 149 to automatically open when the sensor 141 indicates that the wall 18 has been penetrated . if additional pressure is detected by the sensor 141 , the switch 152 would automatically close as illustrated by the arrow 163 . however , the switch 156 would require manual closure by the physician as illustrated by the arrow 165 . in other embodiments , the sensor 141 could be a responsive to the presence or the pressure of insufflation gasses which are commonly used to inflate the abdominal cavity 27 . these gases would be sensed only on the interior side of the wall 18 so the sensor 141 would actually be detecting penetration of the wall 18 by the obturator 36 . these gas pressures could also be sensed in an embodiment providing for a longitudinal channel , such as the axial channel 53 , which could convey the pressures to the sensor 141 at a more proximal location such as that illustrated at the dotted position 167 in fig1 . in still a further embodiment , the sensor 141 could be an infrared sensor including an led 172 activated by the energizer 145 and a detector 174 providing the switch 152 with signal characteristics on the conductor 149 . such an ir sensor 141 as illustrated in fig1 would sense the proximity of tissue by the reflectance of light from the led 172 to the detector 174 . another way of sensing penetration of the wall 18 is particularly adapted for the embodiment illustrated in fig5 and 6 . in this case , the logic circuit 101 provides means for sensing changing electrical characteristics in proximity to the cutting element , such as the blade 93 . these electrical characteristics may include capacitance , resistance , current magnitude , current density , or any combination thereof . these characteristic will tend to vary most dramatically as the blade 93 approaches the interior surface if the wall 18 . as the tissue surrounding the blade 92 is reduced in thickness , the resistance to current flow will rise . not only will the magnitude of the current in conductor 92 decrease , but the density of the current passing through the tissue will also tend to decrease . any one or all of these characteristics can be detected by the logic circuit 101 to provide a means for inhibiting further cutting of tissue upon complete penetration of the wall 18 . the configuration of the distal tip of the trocar 10 as illustrated in fig1 is of particular interest not only to an electrosurgical trocar but also to mechanical trocars . this tip is discussed more specifically with reference to fig1 - 19 . in these figures , structures which are similar to those previously discussed are referred to with the same reference numerals followed by the lower case letter &# 34 ; a &# 34 ;. in fig1 , the obturator 36a is a mechanical obturator adapted for use with a trocar system which cuts tissue with the application of force on a sharp edge of the instrument . the obturator 36a includes a shaft 52a extending along a longitudinal axis 16a between a tip portion 172 at its distal end and a handle portion 174 at its proximal end . the shaft 52a in a preferred embodiment has a generally cylindrical surface 176 . the handle portion 174 includes the finger knob or handle 54a . the distal portion 172 of the shaft 52a is formed from a pair of opposing ramps or surfaces 56a and 58a . these surfaces converge distally and intersect each other at a line 178 which is disposed at the distal end of the tip portion 172 . in a preferred embodiment , the surfaces 56a and 58a are planar in configuration to facilitate manufacture of the device . in other embodiments these surfaces may be more rounded as illustrated in fig1 . the surfaces 56a and 58a are disposed at a relative angle preferably in a range between 15 ° and 45 °. a smaller angle facilitates penetration of the tissue while a larger angle reduces the length of the tip portion 172 . in a preferred embodiment , the relative angle between the surfaces 56a and 58a is about 30 °. this relative angle between the surfaces 56a and 58a can be divided equally by the axis 16a , or alternatively , the surfaces 56a and 58a may have different angles with respect to the axis 16a . in the illustrated embodiment , the line 178 at the intersection of the surfaces 56a and 58a passes through the axis 16a of the shaft 52a . preferably the axis 16a is generally perpendicular to the line 178 and bisects the line 178 . in this embodiment , the line 178 has a length which is generally equivalent to the diameter of the shaft 52a . this configuration for the tip portion 172 is particularly desirable when the obturator 136a is used to puncture the abdominal wall 18a between a pair of ribs 181 , 183 . blood vessels 185 are often present in this region along with various ligaments and other elements of tissue which should not be cut . the wedge configuration of the tip portion 172 enables the obturator 36a to be inserted between the ribs 181 , 183 without exposing the vessels 185 to any sharp cutting edge . furthermore , the opposing surfaces 56a and 58a act to diametrically separate the ribs 181 , 183 so that the only cutting occurs along a plane 187 illustrated by a dotted line which extends perpendicular to the abdominal wall 18a and intermediate the ribs 181 , 183 . it is of particular interest to note that the outer surface 176 of the shaft 52a intersects each of the ramps or surfaces 56a , 58a at an obtuse angle between 90 ° and 180 °. referring to the enlarged view of fig1 one can see that a point a is defined by the intersection of the surfaces 56a , 58a and 176 . at this point a , the surface 176 is generally vertical as shown by the axis 190 . the surface 56a is horizontal as shown by the axis 192 . it follows that an angle 194 defined by the intersection of these surfaces 56a and 176 is about 90 ° at the point a . as the surface 56a moves proximally , its intersection with the surface 176 of the shaft 52a follows generally along a line 196 . from the point a , the line 196 travels proximally and then increasingly radially with progressive positions in the proximal direction . at a point b where the surface 56a intersects the surface 76 at its most proximal position , the line 196 extends only radially . at this point b , the surface 176 extends generally horizontally as shown by an axis 198 . the surface 56a extends almost horizontally along an axis 200 . an angle 202 which exists between an axis 198 and the axis 200 approaches 180 °. in a preferred embodiment wherein the surface 56a has an angle of 15 ° with respect to the axis 16a , the angle 202 is 165 °. it follows that between the points a and b , the angle between the surface 56a and the surface 176 is an obtuse angle which varies generally from 90 ° to 180 °. since this is an obtuse angle , the line 196 is not particularly sharp and therefore is less likely to produce a cutting action . to further inhibit any cutting action along the line 196 , this line of intersection can be rounded to further reduce its sharpness . although the embodiments of fig1 - 19 are directed primarily to a mechanical obturator 36a , it will be apparent that the advantages associated with the tip portion 172 are equally valuable with respect to an electrosurgical trocar . in such an embodiment , the electrosurgical electrode or wire 42 would be disposed along the line 178 . under certain circumstance it may be desirable to view the distal end of the obturator 36a as cutting occurs . such a viewing mechanism might take the form of a fiber optic cable 204 terminating at a wide angle lens 206 as illustrated in fig1 . the viewing mechanism would not only be beneficial in monitoring the cutting of the tissue , but could also provide information as to the nature of the tissue being cut . this would be of particular importance when the obturator 36a fully penetrates the wall 18 . as noted , it is important to immediately cease cutting at this point of penetration in order to avoid any damage to interior organs . although this invention has been discussed with reference to various electrical and optical cutting circuits and exemplary switching circuits , it will be apparent to those skilled in the art that the invention can be otherwise embodied . generally , any energy source can be coupled to the distal end of the obturator 36 and its energy converted into forms suitable for cutting tissue . various cutting elements and cutting element configurations will be apparent depending on the nature of the energy provided . various types of technology can also be incorporated into the sensor 141 all with the intent to provide some measurable indication at the point in time when the obturator 36 penetrates the wall 18 . due to the broad nature of this invention , the breadth of the associated concept should not be limited merely to the disclosed embodiments or the drawings , but should be determined only with reference to the following claims .