Abstract:
In a surgical knife, a slider has an operating part through which the slider is operated. A surface of the operating part is exposed through a window hole of a guide groove and an arresting device for arresting the slider is released by urging the operating part linearly in a direction to be moved. The slider is moved by applying a forward or backward force to the operating part such that a cutter blade of the surgical knife can be moved between its projected position and its withdrawn position with respect to the holder when the holder is held by one hand. Accordingly, a user does not feel pain on his or her finger&#39;s when operating the cutter blade.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a surgical knife to be used mainly in medical institutions. 
   A surgical knife in which a cutter blade is movable between its projected position and its withdrawn position is known in the prior art. For example, in a surgical knife disclosed by JP-B2-2608695, a cutter blade is moved between its projected position and its withdrawn position by rotating an operating part projecting from a surface of a holder and sliding it backward and forward. In a surgical knife disclosed by JP-A-8-24265, a cutter blade is moved between its projected position and its withdrawn position by sliding backward and forward an operating part projecting from an upper surface of a holder. 
   In the surgical knife disclosed by JP-B2-2608695, since it is necessary for moving the cutter blade between its projected position and its withdrawn position that the operating part is rotated and subsequently is moved backward and forward, the cutter blade cannot be moved by one hand between its projected position and its withdrawn position when the holder is held by the one hand, so that this surgical knife is inconvenient for use by the one hand. 
   In the surgical knife disclosed by JP-A-8-24265, the operating part projects from the upper surface of the holder. Since the holder is thin, the operating part projecting from the upper surface of the holder is thinner than the holder. Therefore, an upper end of the operating part bites into a thumb so that a user feels a pain when the thumb is pressed against the operating part to move it. 
   BRIEF SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a surgical knife in which a cutter blade can be moved by one hand between its projected position and its withdrawn position when a holder is held by the one hand, and a user does not feel a pain on a user&#39;s finger when operating the cutter blade. 
   According to the present invention, in a surgical knife comprising, a holder including a guide groove on a front surface of the holder, a slider movable forward and backward in the guide groove, a cutter blade connected to the slider, and an engaging means for fixing the slider to selected one of a front position of the slider in which front position the cutter blade projects from a front end of the holder and a back position in which back position the cutter blade is contained in the holder, 
   the slider has an operating part through which the slider is operated, a surface of the operating part is exposed through a window hole of the guide groove, the engaging means is capable of being released to make the slider movable by pressing the operating part substantially linearly in one direction to be moved, and the slider is movable in accordance with selected one of forward and backward forces applied to the operating part. 
   It is preferable in the present invention that the slider has an elastic part and a slider body, the operating part is swingably mounted on the slider body through the elastic part, and the engaging means is capable of being released to make the slider movable by pressing the operating part in a direction from the front surface of the holder toward a reverse surface of the holder to move the operating part in the direction with a deformation of the elastic part. 
   It is preferable in the present invention that the engaging means has engaging projections formed respectively on upper and bottom surfaces of the operating part and engaging recesses formed respectively on front and back positions of each of upper and lower edges of the window hole of the guide groove so that the slider is fixed by an engagement between the engaging projections and the engaging recesses, and the engaging means is capable of being released to make the slider movable by pressing the operating part in the direction from the front surface of the holder toward the reverse surface of the holder. 
   It is preferable in the present invention that front and back side surfaces of each of the engaging recesses are perpendicular to a longitudinal direction of the holder, and each of the engaging projections has a trapezoidal shape whose width decreases in the direction from the front surface of the holder toward the reverse surface of the holder. 
   It is preferable in the present invention that the slider has an elastic part and a slider body, the operating part is mounted on the slider body through the elastic part movably in a transverse direction perpendicular to a longitudinal direction of the holder, and the engaging means is capable of being released to make the slider movable by pressing the operating part in the transverse direction against an elastic force of the elastic part to move the operating part in the transverse direction with a deformation of the elastic part. 
   It is preferable in the present invention that the slider body and the operating part are separated from each other, the operating part is mounted on the slider body movably in the transverse direction, and the elastic part extends integrally from the slider body in the longitudinal direction to elastically urge the operating part in the transverse direction. 
   It is preferable in the present invention that the operating part has an engaging projection on a lower surface of the operating part, and the guide groove has engaging holes formed respectively on front and back positions of a bottom surface of the guide groove so that the slider is fixed by an engagement between the engaging projection and selected one of the engaging holes. 
   It is preferable in the present invention that the slider has a slider body, the operating part has a hinge at an end thereof to be connected to the slider body through the hinge so that the operating part is swingable in a window hole of the guide groove around the hinge, and the operating part has an engaging projection on one of upper and lower surfaces of the operating part so that the engaging projection engages with an engaging portion of the holder to fix the slider in each of a case in which the cutter blade projects from the holder and a case in which the cutter blade is contained in the holder. 
   It is preferable in the present invention that a part of the operating part extends in the guide groove, and a width of the part of the operating part extending in the guide groove is greater than a width of the window hole. 
   It is preferable in the present invention that the engaging portion is a recess formed on an inner surface of the guide groove, and the operating part is elastically urged toward the engaging portion so that the engaging projection fits smoothly into the engaging portion. 
   It is preferable in the present invention that the hinge urges the operating part toward the engaging portion. 
   It is preferable in the present invention that the slider body has an integrally formed elastic piece to urge the operating part toward the engaging portion. 
   It is preferable in the present invention that the guide groove has a guide hole extending in the longitudinal direction on a central position of a bottom surface of the guide groove, and the slider has a guide projection to be inserted into the guide hole so that a movable range of the slider is limited. 
   In a surgical knife comprising, a holder including a guide groove on a front surface of the holder, a slider movable forward and backward in the guide groove, a cutter blade connected to the slider, and an engaging means for fixing the slider to selected one of a front position of the slider in which front position the cutter blade projects from a front end of the holder and a back position in which back position the cutter blade is contained in the holder, 
   according to the invention, the slider has a slider body and a lever-shaped operating part through which the slider is operated, an end of the operating part is connected to the slider body in such a manner that the operating part is swingable around the end of the operating part, the operating part is capable of engaging with a window hole of the guide groove to fix the slider to the holder when the slider is positioned at each of the front and back positions, and the slider is movable when an engagement between the operating part and the window hole is released by swinging the operating part. 
   It is preferable in the present invention that the guide groove has a recess on each of upper and lower edges of the window hole, the operating part has a projection on each of upper and lower surfaces thereof, and the operating part is swingable by 180 degrees so that the operating part engages with the window hole to fix the slider to the holder with the recess and projection engaging with each other when the slider is positioned at the front position and the operating part engages with the window hole to fix the slider to the holder with the recess and projection engaging with each other when the slider is positioned at the back position. 
   It is preferable in the present invention that the guide groove has front and back recesses distant from each other in a movable direction of the slider on each of upper and lower edges of the window hole, and the operating part has a projection on each of upper and lower surfaces thereof so that the operating part engages with the window hole to fix the slider to the holder with the front recess and the projection engaging with each other when the slider is positioned at the front position and the operating part engages with the window hole to fix the slider to the holder with the back recess and the projection engaging with each other when the slider is positioned at the back position. 
   It is preferable in the present invention that a thickness of the operating part is substantially equal to a thickness of an outer peripheral region of the window hole. 
   According to the present invention, since the surface of the operating part is exposed through the window hole of the guide groove, the engaging means is capable of being released to make the slider movable by pressing the operating part substantially linearly in one direction to be moved, and the slider is movable in accordance with selected one of forward and backward forces applied to the operating part, the cutter blade can be moved by one hand between its projected position and its withdrawn position when the holder is held by the one hand, so that the handling of the surgical knife is easy. Further, since the surface of the operating part is exposed through the window hole of the guide groove, an area of the surface of the operating part can be increased by increasing a width of the window hole, so that a user does not feel a pain on a user&#39;s finger when operating the cutter blade with the finger contacting the operating part. 
   If the slider has the elastic part and the slider body, the operating part is swingably mounted on the slider body through the elastic part, and the engaging means is capable of being released to make the slider movable by pressing the operating part in the direction from the front surface of the holder toward the reverse surface of the holder to move the operating part in the direction with the deformation of the elastic part, and/or if the engaging means has engaging projections formed respectively on the upper and bottom surfaces of the operating part and engaging recesses formed respectively on the front and back positions of each of the upper and lower edges of the window hole of the guide groove so that the slider is fixed by the engagement between the engaging projections and the engaging recesses, and the engaging means is capable of being released to make the slider movable by pressing the operating part in the direction from the front surface of the holder toward the reverse surface of the holder, by increasing the width of the window hole of the guide groove, the user&#39;s finger (for example, thumb) can extend into the window hole when pressing the operating part with the user&#39;s finger. Therefore, it is not necessary for operating the operating part that the surface of the operating part projects by a great length from the surface of the holder. If the surface of the operating part does not project by the great length from the surface of the holder, the cutter blade is prevented from being moved even when the user, for example, a doctor&#39;s smock contacts accidentally the operating part. Further, after being disposed of with the cutter blade being contained by the holder, the operating part is restrained from contacting another member in such a manner that the operating part is operated by the another member to project the cutter blade from the holder. 
   If the front and back side surfaces of each of the engaging recesses are perpendicular to the longitudinal direction of the holder, and each of the engaging projections has the trapezoidal shape whose width decreases in the direction from the front surface of the holder toward the reverse surface of the holder as shown in  FIG. 4 , a force pushing and pulling the cutter blade with respect to the holder when the cutter blade is fixed presses the engaging projection into the engaging recess so that the cutter blade is prevented from moving. 
   If the slider has the elastic part and the slider body, the operating part is mounted on the slider body through the elastic part movably in the transverse direction perpendicular to a longitudinal direction of the holder, and the engaging means is capable of being released to make the slider movable by pressing the operating part in the transverse direction against an elastic force of the elastic part to move the operating part in the transverse direction with the deformation of the elastic part, the operating part can be moved through the surface of the operating part which does not project by the long length from the holder surface and which has an anti-slip texture. Since it is not necessary for moving the operating part that the surface of the operating part projects by the long length from the holder surface, the cutter blade is prevented from being moved even when the user, for example, the doctor&#39;s smock contacts accidentally the operating part. Further, after being disposed of with the cutter blade being contained by the holder, the operating part is restrained from contacting another member in such a manner that the operating part is operated by the another member to project the cutter blade from the holder. Even when the surface of the operating part slightly projects from the holder surface, since the operating part is generally urged perpendicular to the holder surface and is not urged in the transverse direction, the operating part is restrained from being moved undesirably. 
   If the slider body and the operating part are separated from each other, the operating part is mounted on the slider body movably in the transverse direction, and the elastic part extends integrally from the slider body in the longitudinal direction to elastically urge the operating part in the transverse direction, the operating part is linearly moved in the transverse direction so that the user&#39;s finger can smoothly move the operating part. 
   If the operating part has the engaging projection on the lower surface of the operating part, and the guide groove has the engaging holes formed respectively on the front and back positions of the bottom surface of the guide groove so that the slider is fixed by the engagement between the engaging projection and selected one of the engaging holes, the slider can be securely fixed to the holder. 
   If the slider has the slider body, the operating part has the hinge at the end thereof to be connected to the slider body through the hinge so that the operating part is swingable in the window hole of the guide groove around the hinge, and the operating part has the engaging projection on one of the upper and lower surfaces of the operating part so that the engaging projection engages with the engaging portion of the holder to fix the slider in each of a case in which the cutter blade projects from the holder and a case in which the cutter blade is contained in the holder, the operating part and the slider body are formed in one piece by an injection molding so that a production of them is easy. 
   If the part of the operating part extends in the guide groove, and the width of the part of the operating part extending in the guide groove is greater than the width of the window hole, the part of the operating part bent at the hinge is prevented from being removed through the window hole. 
   If the engaging portion is the recess formed on the inner surface of the guide groove, and the operating part is elastically urged toward the engaging portion so that the engaging projection fits smoothly into the engaging portion, the slider can be fixed to the holder automatically for easy handling of the surgical knife. 
   If the hinge urges the operating part toward the engaging portion, the hinge performs two functions as the hinge and elastic part so that a number of elements of the surgical knife is minimized for easy production thereof. 
   If the slider body has an integrally formed elastic piece to urge the operating part toward the engaging portion, the elastic urging force for inserting securely the engaging projection into the engaging portion is increased by the elastic piece. 
   If the guide groove has the guide hole extending in the longitudinal direction on the central position of the bottom surface of the guide groove, and the slider has the guide projection to be inserted into the guide hole so that the movable range of the slider is limited, the slider is prevented from being removed from the holder through a back end of the holder even when the back end of the holder is opened. 
   According to the present invention, since the slider has the slider body and the lever-shaped operating part through which the slider is operated, the end of the operating part is connected to the slider body in such a manner that the operating part is swingable around the end of the operating part, the operating part is capable of engaging with the window hole of the guide groove to fix the slider to the holder when the slider is positioned at each of the front and back positions, and the slider is movable when the engagement between the operating part and the window hole is released by swinging the operating part, it is necessary for moving the cutter blade that the operating part is pulled out of the window hole, so that the cutter blade is prevented from being moved even when the user, for example, the doctor&#39;s smock contacts accidentally the operating part. Further, after being disposed of with the cutter blade being contained by the holder, the operating part is restrained from contacting another member in such a manner that the operating part is operated by the another member to project the cutter blade from the holder. 
   If the guide groove has the recess on each of the upper and lower edges of the window hole, the operating part has the projection on each of the upper and lower surfaces thereof, and the operating part is swingable by 180 degrees so that the operating part engages with the window hole to fix the slider to the holder with the recess and projection engaging with each other when the slider is positioned at the front position and the operating part engages with the window hole to fix the slider to the holder with the recess and projection engaging with each other when the slider is positioned at the back position, a length of the holder may be short. 
   If the guide groove has the front and back recesses distant from each other in the movable direction of the slider on each of the upper and lower edges of the window hole, and the operating part has the projection on each of the upper and lower surfaces thereof so that the operating part engages with the window hole to fix the slider to the holder with the front recess and the projection engaging with each other when the slider is positioned at the front position and the operating part engages with the window hole to fix the slider to the holder with the back recess and the projection engaging with each other when the slider is positioned at the back position, it is necessary for moving the cutter blade that the operating part is pulled out of the window hole, so that the cutter blade is prevented from being moved even when the user, for example, the doctor&#39;s smock contacts accidentally the operating part. Further, after being disposed of with the cutter blade being contained by the holder, the operating part is restrained from contacting another member in such a manner that the operating part is operated by the another member to project the cutter blade from the holder. 
   If the thickness of the operating part is substantially equal to the thickness of the outer peripheral region of the window hole, the operating part in the window hole does not project out of the window hole so that the operating part is prevented from being moved undesirably and the cutter blade can be securely kept stationary. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a schematic oblique projection view showing a surgical knife of a first embodiment of the invention with a cutter blade positioned at its projected position. 
       FIG. 2  is a schematic oblique projection view showing the surgical knife of the first embodiment with the cutter blade positioned at its withdrawn position. 
       FIG. 3  is a schematic oblique projection exploded view of the surgical knife of the first embodiment. 
       FIG. 4  is a schematic oblique projection enlarged view showing a part of an operating part of the first embodiment. 
       FIG. 5  is a schematic oblique projection view showing another connecting structure between the operating part and a holder body in the first embodiment. 
       FIG. 6  is a schematic oblique projection view showing a surgical knife of a second embodiment of the invention with a cutter blade positioned at its projected position. 
       FIG. 7  is a schematic oblique projection view showing the surgical knife of the second embodiment with the cutter blade positioned at its withdrawn position. 
       FIG. 8  is a schematic oblique projection exploded view of the surgical knife of the second embodiment. 
       FIG. 9  is a schematic oblique projection view showing a holder of the second embodiment. 
       FIG. 10  is a schematic oblique projection view showing a surgical knife of a third embodiment of the invention with a cutter blade positioned at its projected position. 
       FIG. 11  is a front view of the surgical knife of the third embodiment in which a cutter blade is removed from the surgical knife and a slider is positioned at its withdrawn position. 
       FIG. 12  is a back view of the third embodiment. 
       FIG. 13  is a bottom view of the third embodiment. 
       FIG. 14  is a front exploded view of the third embodiment. 
       FIG. 15  is a back view of a slider of the third embodiment. 
       FIG. 16  is a cross sectional view of the third embodiment taken along an imaginary plane A-A in  FIG. 11 . 
       FIG. 17  is a schematic oblique projection view showing a surgical knife of a fourth embodiment of the invention with a cutter blade positioned at its projected position. 
       FIG. 18  is a schematic oblique projection view showing the surgical knife of the fourth embodiment with the cutter blade positioned at its withdrawn position. 
       FIG. 19  is a schematic oblique projection exploded view of the surgical knife of the fourth embodiment. 
       FIG. 20  is a schematic oblique projection view showing a connecting structure among an operating part, a holder body and so forth in the fourth embodiment. 
       FIG. 21  is a combination of an oblique projection view and a cross sectional view showing a projecting portion of the operating part of the fourth embodiment. 
       FIG. 22  is a schematic oblique projection view showing a surgical knife of a fifth embodiment of the invention with a cutter blade positioned at its projected position. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A first embodiment of surgical knife as shown in  FIGS. 1-4  corresponds to the claimed inventions as recited in claims  1 - 4 . A guide groove  2  is formed on a front surface of a holder  1 , and a slider  3  is supported in the guide groove  2  to be movable forward and backward. A cutter blade  4  is connected to the slider  3  in such a manner that the cutter blade  4  projects from a front end of the holder  1  when the slider  2  is positioned at its front position, and the cutter blade  4  is completely contained in the holder  1  when the slider  2  is positioned at its backmost position. 
   The slider  3  includes a slider body  5 , an operating part  6  and an elastic part  7 . The operating part  6  and elastic part  7  are formed in one piece, and a base part  8  formed integrally with the operating part  6  and elastic part  7  is fixed to the slider body  5  so that the operating part  6  is connected to the slider body  5  through the elastic part  7 . Since the elastic part  7  extends obliquely with respect to a longitudinal direction of the surgical knife, the operating part  6  is movable in a direction perpendicular to the front surface of the holder  1 . The base part  8  includes projections  9  fitted in respective holes of the slider body  5  to be fixed permanently. As shown in  FIG. 5 , elastic pieces  15  of L-shaped cross-section may extend from a back end of the holder body  5  so that the base part  8  may be received between the elastic pieces  15  while engaging recesses of the base part  8  engage with engaging protrusions  50  of the holder body  5 , so that the base part  8  is fixed to the slider body  5 . 
   Upper and lower surfaces of the operating part  6  have engaging projections  11  facing to the front surface of the holder  1 . The engaging projections  11  have respective trapezoidal shapes each of which has a width decreasing in a direction from the front surface of the holder  1  toward a reverse surface of the holder  1 . 
   An elongated window hole  12  is arranged on the front surface of the holder  1  and extends to the guide groove  2 . The operating part  6  is movable is movable in the window hole  12  forward and backward. A back end of the window hole  12  has engaging recesses  13  at upper and lower edges of the window hole  12  respectively, and a front end of the window hole  12  has engaging recesses  14  at the upper and lower edges of the window hole  12  respectively. When the slider  3  is positioned at its backmost position, the engaging projections  11  engage with the engaging recesses  13  at the back end of the window hole  12 , and when the slider  3  is positioned at its front end position, the engaging projections  11  engage with the engaging recesses  14  at the front side of the window hole  12 . 
   The slider  3  is moved by urging the operating part  6  forward and backward after pushing a surface of the operating part  6  to release the engaging projections  11  from the engaging recesses  13  or  14 . 
   A second embodiment of surgical knife is shown in  FIGS. 6-9 . Similarly to the first embodiment, the guide groove  2  is formed on the front surface of a holder  1 , and the slider  3  is supported in the guide groove  2  to be movable forward and backward. Further, similarly to the first embodiment, the cutter blade  4  is connected to the slider  3  in such a manner that the cutter blade  4  projects from the front end of the holder  1  when the slider  2  is positioned at its front position, and the cutter blade  4  is completely contained in the holder  1  when the slider  2  is positioned at its backmost position. 
   The operating part  6  and the slider body  5  are separated from each other. The slider body  5  includes a transverse groove  17  in the vicinity of a central position of the slider body, and the elastic part  7  extends over the transverse groove  17  in a longitudinal direction of the slider body  5  as a cantilever. A core part of the operating part  6  fits in the transverse groove  17  in such a manner that the operating part  6  is movable transversely along the transverse groove  17 . The operating part  6  has a downward extending engaging projection  19  for fixing the operating part  6 . The elastic part  7  contacts an upper surface of the core part of the operating part  6  to urge elastically the operating part  6  downward. 
   The cutter blade  4 , the slider body  5  and the operating part  6  fitting in the transverse groove  17  of the slider body  5  are received by the guide groove  2 . When the slider  3  is positioned at its backmost position so that the cutter blade  4  is completely contained by the holder  1 , the engaging projection  19  fits in an engaging hole  20  on a bottom surface of the guide groove  2  at a back side of the guide groove  2  so that the slider  3  is fixed. Since the operating part  6  is urged downward by the elastic part  7 , this fixed condition of the slider  3  is stable. 
   When the slider  4  is positioned at its front end position so that the cutter blade  4  projects from the front end of the holder  1 , the engaging projection  19  fits in an engaging hole  21  at a front side of the guide groove  2  so that the slider  3  is fixed. The slider  3  is moved by urging the operating part  6  forward and backward after pushing upward the surface of the operating part  6  to release the engaging projection  19  from the engaging hole  20  or  21 . 
   A third embodiment of surgical knife is shown in  FIGS. 10-16 . Reference numerals R and L indicate a radial direction and longitudinal direction of the window hole, respectively. As shown in  FIGS. 14 and 15 , the slider  3  is formed in one-piece by plastic injection molding. The slider  3  has the slider body  5  and the operating part  6 . A cutter blade connecting protrusion  37  is arranged on a front end of the slider body  5 , and a rectangular cut-off portion  38  is arranged at a side of the slider body  5  on a back half of the slider body  5 . A guide piece  39  extends backward from a front side of an inner surface of the cut-off portion  38 , and a terminating end of the guide piece  39  has a guide projection  40 . An elastic piece  41  extends forward from a backside of the inner surface of the cut-off portion  38 , and a terminating end of the guide piece  41  has a projection  42 . 
   The operating part  6  is swingably connected through a hinge  43  to the slider body  5  in such a manner that the operating part can extend obliquely with respect to the slider body  5 . As shown in  FIG. 16 , a width of the operating part  6  in the window hole  12  is narrower than a width of the window hole  12  so that the operating part  6  is movable in the window hole  12 . A width of base portions  44  of the operating part  6  is wider than a width of the window hole  12  and the base portions  44  fit in the guide groove  2  so that the operating part  6  is prevented from slipping out of the window hole  12  to the near side. Reference numerals R and OP indicate a radial direction and opening direction of the window hole, respectively. 
   A side wall  45  extends integrally from a side edge of one of the base portions  44  in a direction perpendicular to the base portions  44 . The hinge  43  is a thin portion of the side wall  45 . The side wall  45  has an engaging protrusion  46  on an outer surface of the side wall  45 , so that the engaging protrusion  46  engages with one of engaging recesses  47  and  48  to fix the slider  3 . 
   When the engagement between the engaging protrusion  46  and the one of engaging recesses  47  and  48  is released and the slider  3  is moved in the guide groove  2 , the operating part  6  is displaced upward by a distance corresponding to a protruding height of the engaging protrusion  46 . By the upward displacement of the operating part  6 , the side wall  45  contacts the projection  42  of the elastic piece  41 , and by a reaction force against a deformation of the elastic piece  41 , the operating part is urged in a counter direction. Whereby when the slider  3  is moved so that the engaging protrusion  46  reaches the engaging recess  47  or  48 , the engaging protrusion  46  is securely fitted into the engaging recess  47  or  48  by a strong elastic force of the elastic piece  41 . 
   The holder  1  of this embodiment has a guide hole  49  extending in a longitudinal direction of the guide groove  2  on a central position of a bottom surface of the guide groove  2 . The guide hole  49  receives the guide projection  40  of slider  3  so that the slider  3  slides smoothly. When the slider  3  is fixed in the guide groove  2 , that is, the engaging protrusion  46  fits in the engaging recess  47  or  48 , the guide projection  40  is positioned at one of ends of the guide hole  49 . Therefore, the slider  3  can be stopped to securely fit the engaging protrusion  46  in the engaging recess  47  or  48 , even when the slider  3  is moved in a high speed. Although a back end of the holder  1  is opened in such a manner that the slider  3  is removed from the holder  1  through the back end thereof, the movement of the guide projection  40  is limited by a back end of the guide hole  49  so that the slider  3  is prevented from being removed from the holder  1  through the back end thereof. 
   A fourth embodiment of surgical knife as shown in  FIGS. 17-22  corresponds to the claimed inventions as recited in claims  14 - 17 . Similarly to the above embodiments, the guide groove  2  is formed on the front surface of a holder  1 , and the slider  3  is supported in the guide groove  2  to be movable forward and backward. Further, similarly to the above embodiments, the cutter blade  4  is connected to the slider  3  in such a manner that the cutter blade  4  projects from the front end of the holder  1  when the slider  2  is positioned at its front position, and the cutter blade  4  is completely contained in the holder  1  when the slider  2  is positioned at its backmost position. 
   The operating part  6  of a slightly elongated plate shape has a pair of pin supporting pieces  22  on a base end thereof. The pin supporting pieces  22  have respective pin holes  23 . The operating part  6  has projections  24  on respective upper and lower surfaces of the operating part  6  to fix the slider. 
   The slider body  5  has a pin-receiving hole  25  on a surface thereof. The operating part  6  is swingably supported by the slider body  5  by arranging the pin receiving hole  25  between the pin supporting pieces  22 , and inserting a pin  26  through the pin holes  23  and the pin receiving hole  25  with fixing the pin  26 . 
   As shown in  FIG. 20 , the pin receiving hole  25  is formed by that base plates  31  of a pin receiving piece  30  forming the pin receiving hole  25  are inserted into an enlarged portion of a stepwise rectangular hole  32  of the slider body  5  in such a manner that a U-shaped portion of the pin receiving piece  30  extends through the rectangular hole  32  to project from a surface of the slider body  5 . 
   Recesses  27  for receiving therein the projections  24  are formed on respective upper and lower edges of the window hole  12  of the holder  1 . When the cutter blade  4  is projected from the front end of the holder  1 , as shown in  FIG. 17 , the base end of the operating part  6  is positioned at the front end of the window hole  12  and is fitted in the window hole  12 . At this time, since the projections  24  are fitted in the recesses  27  respectively, the slider  3  is prevented from moving forward and backward. A space  28  is used to facilitate pulling up the operating part  6  when the operating part  6  is reversed. As shown in  FIG. 21 , each of the projections  24  may have an engaging protrusion  34  extending over a slit  33  at an outside of the projection  24 , and each of the recesses  27  may have an engaging recess (not shown) for engaging with the engaging protrusion  34  on an inner surface of the recess  27 , so that an engagement between the engaging protrusion  34  and the engaging recess prevents securely the operating part  6  from moving out of the window hole  12 . 
   If a thickness of the operating part  6  is substantially equal to a thickness of a window hole peripheral area  29  of the guide groove  2  as recited in claim  17 , the operating part  6  is prevented from protruding from the surface of the holder  1 , so that the slider  3  is securely prevented from being moved undesirably. 
   When the cutter blade  4  is completely contained by the holder  1 , the operating part  6  is pulled up to release the engagement between the recesses  27  and the projections  24  in a condition shown in  FIG. 17 , subsequently the operating part  6  is moved to position the base end of the operating part  6  at the backmost position of the window hole  12 , and finally the operating part  6  is fitted into the window hole  12  to insert the projections  24  into the recesses  27 . 
     FIG. 22  shows a fixing method of the slider  3  as recited in claim  16  as a fifth embodiment. The cutter blade  4 , the slider body  5 , the operating part  6  and a swing mechanism between the slider body  5  and the operating part  6  of this embodiment are the same as those of the fourth embodiments. 
   The window hole  12  of this embodiment is longer than that of the fourth embodiment, a pair of recesses  35  at a back side of the window hole  12  and a pair of recesses  36  at a front side of the window hole  12  correspond to the recesses  27  of the fourth embodiment. When the cutter blade  4  is projected from the front end of the holder  1 , the projections  24  are fitted in the recesses  36  at the front side of the window hole  12 . When the cutter blade  4  is contained in the holder  1 , the projections  24  are fitted in the recesses  35  at the backside of the window hole  12 .