FIELD OF INVENTION 
     The present invention relates generally to the field of surgical instruments and more particularly to a scalpel type cutting instrument which is designed to be manipulated by the hand of a physician. 
     BACKGROUND OF THE INVENTION 
     Surgical cutting instruments such as scalpels are widely used to perform surgery. A variety of blade configurations are widely available. The traditional scalpel consists of a blade fixed to a handle. In operation the handle is manipulated by the physician to make an incision. Although infrequent the exposed cutting surface of the traditional scalpel can inadvertently cut the physician or surgical assistant. The potential for blood born infection has created a demand for surgical instruments which retract or otherwise shield the user from inadvertent “pricks”. For example a wide variety of scalpels with retractable blades are available in the industry. See for example U.S. Pat. No. 5,403,337. Although “safe” surgical instruments are now widely available there is a continuing need to improve the performance of such devices so that they meet the users&#39; expectations in terms of performance as a surgical instrument as well as offer improved safety. 
     SUMMARY OF INVENTION 
     In contrast to prior art devices the present cutting tool incorporates a number of features. In one embodiment the blade may be rotated about its major axis providing a variety of cutting angles for a given handle position. In another embodiment the blade orientation is fixed. 
     In all embodiments the blade retracts into the handle which defines the closed position. The blade is biased toward a closed configuration by a spring feature which may be activated by a finger release. When the finger release is activated the blade is automatically retracted into the body of the device. To move the blade into the “open” or operating position the user moves the finger slider toward the proximal end of the handle and a combination of gears and racks advances the blade out of the distal tip of the body. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     Throughout the several figures of the drawings identical reference numerals refer to identical structure throughout. An exemplary form of the device is shown in the figure and numerous departures may be made without departing from the scope of the invention wherein; 
     FIG. 1 is a respective view of the device in use; 
     FIG. 2 is a perspective view of the device with the blade in the “open” operating position; 
     FIG. 3 is figure showing a partial view of a portion of the scalpel; 
     FIG. 4 is a perspective view of a laparoscopic version of the device; 
     FIG. 5 is a cross-section of the device is a figure with the blade in a retracted “closed” position; 
     FIG. 6 is a cross-section of the device with the blade in an intermediate position; and, 
     FIG. 7 is a cross-section of the device with the blade in the fully advanced or “open” position. 
    
    
     DETAILED DESCRIPTION 
     In FIG. 1 the surgical tool or device  10  is being held in a users hand  12 . The index finger  13  is on the finger slider  14  which is used to advance the blade  15  out of the body  16 . The blade emerges from the distal end  18  of the body  16  and the body  16  forms a handle that rests comfortably in the hand of the user with the proximal end  20  of the body  16  cradled in the “crook” of the hand. 
     FIG. 4 shows an extended version of the device for use in a laparoscopic surgical procedure. The nose  15  of the body member is extended for a length of several inches and the outer surface is round to make the tool compatible with surgical ports. It is anticipated that the interior portion of the body member will be sealed with an o-ring or the like to prevent gas from passing through the instrument. 
     FIG. 2 is a perspective view of the device in isolation. It shows the axis  24  which extends the length of the body  16 . The figure shows the slider  14  which may be advanced or retracted along the path  26  which is preferably parallel to the axis  24 . The finger slider carries at its rear most portion a blade release  28  which may be conveniently operated by the index finger when actuation is intended. 
     It is preferred to incorporate the blade release  28  into the moveable finger slider  14 . The blade release  28  may be used to release the blade from a locked condition. The blade may retract quickly and automatically if the index finger is removed from the slider or the user&#39;s finger may be used to control the retraction of the blade into the body. In the preferred embodiment the blade is biased into the retracted position, although unbiased versions may be preferred by some users. 
     FIG. 3 is a partial view of the distal end of the device showing an embodiment where the blade may be rotated as depicted by arrow  13  around the central axis. Some physicians prefer the ability to cut at an angle. 
     FIGS. 5,  6 , and  7  should be considered together as they represent various stages of the operation of the device and its features. Compared to FIG. 5 the body orientation is reversed in FIG. 6 to illustrate additional components more clearly. 
     In the preferred form of the invention the blade  15  is mounted to an elongate rail  30  which includes a blade rack  32  element. The rail  30  is a close fit with the body  16  so that the blade is held securely in the body when the blade is in the advanced position seen in FIG.  7 . 
     The slider  14  reciprocates along the path  26  indicated by the arrow above the body. In FIG. 6 the slider  14  incorporates a slider rack element  36  which operates a small pinion gear  38  located on an axle  40 . This axle  40  is journaled within the body  16  along axle axis  42  best seen in FIG.  2 . 
     Linear motion imparted to the slider  14  is stepped up by the large pinion  48  which engages the blade rack  32 . Motion of the blade  52  out of the body stretches spring  50 . The overall gear ratios result in a 1:3 in the drawing of figures. The relationship between slider motion and blade motion although the direction is reversed in the sense that retrograde motion of the slider advances the blade. Other gear ratios are possible but ratios greater than between 1:1 are preferred with a ratio of and 1:3 preferred when used with a spring. 
     The cantilevered stop  56  feature engages a boss  58  formed in the body  16  to hold the blade holder  52  in the advanced and “locked” position against the retraction force of the spring  50 . The detent mechanism formed by blade release  28 , boss  58  and lever  56  cooperate to hold the blade open as best seen in FIG. 7 can be overcome by depressing the blade release  28  portion of the slider  14  which dislodges the cantilevered beam  56  from the boss  58 . At this point the blade holder  52  and associated blade  15  are retracted under spring force and its motion may be controlled by the user&#39;s finger on the slider  14 . Although a helical metal spring is shown in the figure for convenience it must be appreciated that an elastic band made of rubber of the like may readily substituted. In a similar fashion a “plastic” spring member made of the parent material is contemplated within the scope of the invention and are encompassed with the term spring member. 
     The comfort and utility of the tool depends in part on its&#39; “feel”. It is expected that adding weight to the tool will make it more acceptable to some physicians. The ballast  57  can be added to increase the weight of the tool. By placing the ballast off the center line of the tool the asymmetry of the design and the ballast cooperate to prevent the tool from rolling on an inclined surface. This is regarded as a desirable feature of the design.
The present invention is a scalpel with a retractable blade which may be advanced or retracted by operation of a slider. The scalpel has multiple operating positions and the physician may conveniently retract the blade by actuating the slider.