reference will now be made in detail to the present preferred embodiment of the invention , an example of which is illustrated in the accompanying drawings . a preferred embodiment of the present invention comprises a nerve stimulator function that allows control of the current output through a fingertip control on the stimulating needle . a preferred embodiment of the needle assembly of the present invention is illustrated in fig1 a ( front view ) and fig1 b ( side view ). a needle assembly consists of a housing unit 102 . the housing unit may be made of any suitable material such as molded plastic . the housing unit 102 preferably contains a plurality of electrical pin connectors 103 , 104 and 105 that electrically connect the housing unit 102 to an external nerve stimulator device or a plurality of external devices . electrical traces 139 , 140 , and 143 are preferably embedded within the housing unit 102 and are electrically connected to a corresponding electrical pin connector . the housing unit 102 also preferably contains an embedded fluid path 110 through which fluid from a tube 109 may flow to a needle 107 . the tube 109 may be any tube suitable for carrying fluids such as a plastic injection tube . the tube 109 may be formed within the housing unit 102 or may mate with the housing unit 102 via known techniques of mating tubes . needle 107 is preferably inserted into a cavity ( not shown ) in the housing unit 102 so that it mates with the embedded fluid path 110 . needle 107 may be detachable from housing 102 or may be permanently affixed to housing 102 . those of skill in the art will appreciate that the tube 109 and the needle 107 should be attached to the embedded fluid path 110 according to techniques known in the art in a manner that avoids leakage of the fluid and also avoids contamination of the fluid . the hypodermic needle preferably consists of stainless steel , and is preferably coated with an insulation layer 108 , although a needle without the insulation layer may be used but may provide less efficient current transfer to the nerve . the needle tip is preferably not coated with the insulation layer and the needle tip may be of any type of bevel , such as a short or a long bevel . the insulation layer 108 is preferably a biocompatible insulation layer and preferably comprised of teflon , polyethylene , pvc , polypropylene , or any other suitable material . a variable control mechanism 101 for variably controlling the current applied to needle 107 may include any type of finger actionable switch , such as a rocker switch , pressure switch , slider switch or any other known finger actionable switch , attached to the housing unit 102 . a voltage may be applied from a voltage source 180 to the variable control mechanism 101 via an electrical pin connector 103 and electrical trace 140 in the housing unit 102 and reduced or increased depending on the operation of the variable control mechanism 101 . the output voltage on the variable control mechanism 101 is directed to a control device 181 , via a second electrical pin connector 104 , for processing and determining the output current to be delivered from a current source 182 via the third electrical pin connector 105 to the embedded hypodermic needle 107 . the current provided by the current source 182 is preferably in the form of a pulse train as known in the art . fig2 illustrates the operation of the stimulator needle assembly . as shown in fig2 , a syringe 132 is connected to housing unit 102 via tube 109 . a hypodermic needle 107 is also attached to the housing unit 102 , and advanced through the skin surface 136 . a nerve stimulator device 131 is electrically connected to electrical pin connectors 103 – 105 via electrical cables 126 , 142 , and 122 through electrical pin connectors 129 , 130 and 127 , respectively . the source 180 , control device 181 and the current source 182 are contained in the nerve stimulator 131 . as the needle is advanced through the skin surface 136 , the nerve stimulator 131 is activated and controlled by the variable control mechanism 101 via the current output control cable 142 attached to the nerve stimulator 131 by electrical pin connector 130 . the output current is supplied through electrical pin connector 127 via an electrical cable 122 to electrical pin connector 105 on the housing unit 102 . voltage is output through electrical pin connector 129 via electrical cable 126 to electrical pin connector 103 on the housing unit 102 for output current control . the electrical return electrode 135 bears a connector 134 that attaches via an electrical cable 123 to an electrical pin connector 128 on the nerve stimulator 131 . the return electrode 135 is typically a silver - silver chloride electrocardiographic electrode . the operator is able to determine the proximity of the needle tip to the nerve by observation of a visible twitch stimulated in the muscle supplied by the target nerve . in a nerve stimulator operation , current is supplied to the hypodermic needle 107 at a level of 1 – 2 ma and the needle advanced until muscle twitch is achieved . by following appropriate muscle twitches , the amplitude of the current output may be gradually decreased by the use of the variable control mechanism 101 until twitch is observed at less than 0 . 5 ma as shown on display 141 . when the needle tip position is an appropriate distance from the nerve , e . g ., 1 – 2 mm at a current output of less than 0 . 5 ma , the operator injects the solution in the syringe . this process , using a nerve stimulator needle of the prior art , is generally described in u . s . pat . no . 5 , 830 , 151 to hadzic et al . a second embodiment of the present invention is illustrated in fig3 a , 3 b and 3 c . the second embodiment is similar to the first embodiment except that the second embodiment includes the ability to determine the depth of the needle inserted in the skin . the same components are numbered with the same reference numbers . fig3 a illustrates a frontal view of the second embodiment , fig3 b illustrates a side view , and fig3 c illustrates a cross sectional view of the needle of the second embodiment . the needle 107 shown in fig3 a , 3 b and 3 c is able to determine depth of hypodermic needle penetration beneath the skin by using a linear resistance coating 106 connected to the voltage source 180 via an electrical pin connector 103 and an electrical trace 111 . the preferred material for this linear resistance coating is a conductive polymer coating such as a polyaniline ( ormecon ™), although any suitable linear resistive material may be used , such as a normally nonconductive polymer that has been doped with a conductive material . for example , the nonconductive polymer silastic can be doped with carbon to become conductive . by controlling the amount of dopant , the resistance of the polymer may be adjusted to suitable levels . the linear resistance coating 106 is separated from the stainless steel hypodermic needle 107 by an insulation layer 108 , shown as a hatched area surrounding the needle in the figures . the needle of the second embodiment also may be used with the nerve stimulator 131 ′ shown in fig4 . the needle of the second embodiment locates a nerve in the same manner as described in the first embodiment , except that the needle of the second embodiment has the ability to determine the insertion depth of the needle in the skin of a subject . for this reason , nerve stimulator 131 ′ in fig4 contains a constant current voltage source 190 . existing nerve stimulators are designed to deliver constant current pulses for nerve stimulation purposes . these devices do not provide constant current ( a constant non pulsed current ). however , one of skill in the art is able to readily design a constant current source 190 , and the circuit design may be analogous to that for producing the constant current pulse for stimulation . the nerve stimulator 131 ′ described herein preferably contains two separate current sources , one of which , the constant current source 182 , is adjustable by the user for stimulation pulse generation , i . e . pulse current i . the other of which , the constant current voltage source 190 , is not adjustable by the user and provides a constant current ( i ) to the linear resistance coating 106 . by way of example , as shown in fig4 , the return electrode 135 for depth determination by resistance measurement is located remotely on the skin surface and is the same return electrode as that for the current output of the nerve stimulator 131 . in operation , a voltage signal from the constant current voltage source 190 is applied to the linear resistance coating 106 via electrical pin connector 103 and electrical trace 111 . the circuit is completed through the linear resistance coating 106 as it penetrates the skin , thus the return electrode 135 detects the signal and provides the detected signal to the nerve stimulator 131 ′ via electrical cable 123 and electrical pin connector 128 . the nerve stimulator 131 ′ detects the voltage of the detected signal using a voltmeter 145 . the resistance of the linear resistance coating 106 is constant per unit length and significantly higher than that of tissue , which is on the order of 1 . 0 megaohm per mm . tissue impedance is typically in the range of 0 . 1 – 1 . 0 kilohms . since the tissue impedance is less than the resistance of any portion of the needle by orders of magnitude , the resistance of the circuit is approximately that of the needle coating alone . the total resistance ( r t ) of the needle linear resistance coating 106 is the product of the resistance per unit length ( r l ) and the length of the needle ( l t ). as the needle is advanced through the skin , r t may be represented as the sum of the length protruding above the skin ( l a ) 137 multiplied by r l and of the length beneath the skin ( l b ) 138 multiplied by r l , as given in equation 1 . r t = r l × l t =( r l × l a )+( r l × l b ) eq . 1 therefore , since the resistance determined by the nerve stimulator 131 ′ in this circuit is directly related to the length of the coating that protrudes above the skin surface ( r a = r l × l a ), the length of the needle below the skin surface may be determined by equation 2 . l b =( r t − r a )/ r l eq . 2 where r a is the resistance of the portion of the needle protruding above the skin . since r t and r l are known and r a is calculated directly from the ratio of the measured voltage to the applied current ( ohm &# 39 ; s law : v = ir ), l b may be calculated . if the needle is inserted to the point that the housing unit 102 contacts the skin surface , l a = 0 , the measured resistance is that of the tissue alone , and thus l b = l t in that situation . the return electrode 135 is used for both stimulator current control and for needle depth determination through the measurement of resistance . fig7 illustrates an exemplary method of determining the depth of the needle which is preferably carried out by a microprocessor in the nerve stimulator 131 ′. as shown in step s 0 , the nerve stimulator 131 ′ illustrated in fig4 provides a constant dc current i , by way of a variable voltage , to the linear resistive coating 106 on the nerve stimulator needle 107 via electrical pin connector 103 and electrical trace 111 . the nerve stimulator 131 ′ provides a periodic current pulse i to the needle 107 , having its amplitude controlled by variable control mechanism 101 , via electrical trace 139 ( step s 2 ). the constant current i is preferably continuously provided , and is not interrupted by the periodic current pulse i provided to the needle 107 . the resistance r a is determined from the dc current i after the current pulse i decays in the skin of the subject , i . e . during the portion of the interpulse interval when no current pulse from the stimulator function is occurring . the resistance measurement cannot occur early in the interpulse interval due to capacitive functions of the skin which result in a discharge voltage according to the relationship v = v i × e (− t / rc ) , where v is the observed voltage , v i is the applied voltage of the current pulse i on needle 107 , t is the elapsed time , and rc is the product of the resistance and the capacitance of the skin . consequently , the resistance measurements for depth determination are collected in a timed fashion , rather than continuously . for example , in the preferred embodiment , the current pulse is less than or equal to 5 msec in duration and , since the frequency of the pulse signal provided to the nerve stimulator needle 107 is a maximum of 5 hz , the interpulse duration is 45 msec . in step s 4 , the microprocessor determines the decay time t of the current pulse in the skin of the subject as t = 5rc . particularly , for a circuit containing a parallel resistance ( r ) and capacitance ( c ), such as biological tissue , a tissue time constant may be calculated from the product of r and c . when subjected to a current pulse , the capacitive element of such a circuit charges over a defined time interval . on termination of the current pulse , this capacitive element discharges over a characteristic time interval , which follows an exponential decay curve , which takes about five tissue time constants ( 5rc ) to reach 99 % of the final value . this is described in more detail by nunn ( nunn j f , applied respiratory physiology , butterworths , london , 1977 , pg . 464 – 469 ) and horowitz and hill ( horowitz p and hill w , the art of electronics , cambridge university press , cambridge , mass ., 1986 , pg . 20 – 21 ). since tissue time constants are in the range of 1 msec or less , allowing greater than 5 msec to pass following the termination of the stimulating pulse provides adequate time for the measured voltage to approximate baseline values of the voltage applied to the resistive layer . alternatively , t may set to a value greater than or equal to 5 msec . at step s 6 , the microprocessor waits for the time t to elapse since the end of the current pulse i ( no in step 6 ). once the time t has elapsed ( yes in step 6 ), the measured voltage from return electrode 135 via voltmeter 145 is used to calculate r a ( step s 8 ). the needle depth l b is then calculate in step s 10 according to equation 2 , and displayed in step s 12 . the calculated value of l b may then be visually displayed on display 141 . those of skill in the art will appreciate that the calculated value l b may also be audibly displayed and / or may be printed by a printing device attached either directly or indirectly to nerve stimulator 131 ′. fig5 illustrates a cross section of the needle that may be used with the second embodiment . as illustrated in fig5 , needle 107 is coated with insulation layer 108 that is coated with linear resistive coating 106 as in the second embodiment . in fig5 , indelible marks 113 , preferably comprised of biocompatible material , may be included on the resistance coating to provide visual reference of the depth to which the tip of the needle has been inserted . these depth measuring mechanisms allow accurate recording of the needle depth at which an injection was performed and allow for accurate maintenance of needle position during the injection procedure as well as for a period of time after injection when subsequent stimulation attempts are undertaken by the anesthetist . it will be appreciated that the indelible marks 113 may be applied to the insulation layer 108 , in the absence of the resistive coating 106 , to provide a visual depth reference . a third embodiment is shown in fig6 . in this embodiment , a strip of resistance material 112 , such as a tantalum wire , or ceramic strip resistance , may be substituted for the conductive polymer . this material preferably runs the length of the needle shaft and is separated from the hypodermic needle 107 by the insulation layer 108 . this material provides the same linear resistance characteristics as the conductive polymer for use in depth determination , and operates in the same manner as described in embodiment 2 . a fourth embodiment of the invention is illustrated in fig8 and 9 . in this embodiment , the variable control mechanism 101 ″ may be an optical device controlling the current output by a mixture of light wavelengths determined by the degree to which the variable control mechanism 101 is depressed . an example of such a control device is the coldswitch ™. in the case of optical control , the electrical pin connector 104 is replaced by a fiber optic cable 124 . the same elements in this embodiment as in the previous embodiments are numbered the same . just as in the first embodiment , the variable control mechanism 101 ″ is preferably connected to a nerve stimulator 131 or 131 ′ that senses the direction of change mediated through the variable control mechanism 101 ″, as well as the rate of change of switching events . the operation of this embodiment is illustrated in fig9 . in fig9 , light from an led source 116 contained in a photonic sensor and control 115 is directed via fiber optic cable 124 to an optical control mechanism 114 , contained in the variable control mechanism 101 ″. optical control mechanism 114 contains a reflective plate 118 that is partially colored , such as by a color coating , or is a graduated reflective plate that has different reflective properties along the length of the plate . optical control mechanism 114 also contains a pivot 119 upon which reflective plate 118 may pivot upon the application of pressure from an operator . the light from led source 116 is reflected off of reflective plate 118 and transmitted back to the photonic sensor 117 via fiber optic cable 124 . the photonic sensor and control 115 detects the direction of change of variable control mechanism 101 ″, mediated through the optical control mechanism 114 , as well as the rate of change of switching events , by the reflected color or intensity mixing . color or intensity mixing is accomplished by a reflective plate 118 . when the reflective plate 118 is rocked by finger pressure on its pivot 119 , the white incident light emitted from the fiber optic cable 124 is reflected back with altered color or intensity components . the intensity or color is detected by the photonic sensor 117 , electronically processed by the microprocessor 120 and converted to a corresponding current output via the constant current pulse generator 121 . the current output is directed to the stimulating needle electrical pin connector 105 by an electrical cable 122 with the return supplied by a second electrical cable 123 to the return electrode 135 located remotely on the skin surface . just as in the first embodiment , commands to the nerve stimulator can be controlled through switching events , including , but not limited to sequential taps , sudden release or sudden depression . the foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description . it is not intended to be exhaustive or to limit the invention to the precise form disclosed , and obviously many modifications and variations are possible in light of the above teaching . for example , although the method of fig7 is described as being performed by a microprocessor , the method may be performed by a hard wired system or any other suitable processing system . additionally , the terms electrical traces and electrical cables is are considered to encompass any type of known electrical conductors , such as , but not limited to metallic wires or non metallic electrical conductors , which may be embedded or non - embedded , and which may be coated with an insulator or non - coated .