together by suitable conventional means such as screws 69 in either case the lower portion of the trigger arm 66 is provided with a finger grip 70 for actuating the trigger arm 66 the finger grip 70 is preferably formed as an elongate opening through the trigger arm to aid the user not shown in grasping the instrument 10 the upper portion of the trigger arm 66 is formed as a yoke having arms 71 and 72 extend on either side of the receiver portion 15 of the handle 11 as best seen by referring to fig1 and 5 an aperture 71a is formed through the yoke arm 71 similarly an aperture 72a is formed through the yoke arm 72 the purpose of the apertures 71a and 72a will be explained below a pin 73 extends between the yoke arms 71 and 72 and through the receiver portion 15 of the handle 11 to pivotally mount the trigger arm 66 on the handle 11 for movement relative thereto a bore 74 extends vertically through the upper portion of the trigger arm 66 extending from between the yoke arms 71 and 72 to a central recess 75 formed in the distal face of the trigger arm 66 a second central recess 76 is formed on the proximal face of the trigger arm 66 the purpose of the bore 74 and of the recesses 75 and 76 will be explained below a spring 77 or similar resilient structure is provided for urging the trigger arm 66 to a first position relatively away from the grip portion 14 of the handle 11 the first position of the trigger arm 66 corresponds to the unactuated condition of the tool 13 as will be further explained below when the trigger arm 66 is moved toward.