Abstract:
Apparatus and methods are disclosed for holding a subject body (or body portion, generally termed “body”) at a desired stereotaxic orientation relative to a known reference point, in three-dimensional space, where a reference X-axis, a reference Y-axis, and a reference Z-axis mutually intersect. The reference point can be co-positioned with a target point on or in the subject body so as to place the body in a reference position used in a corresponding anatomical atlas or other locational index. With the body so positioned, a probe or other tool can be inserted into the body to a desired locus with high accuracy (in hitting the desired locus) and with high precision (from one animal to the next). The methods and apparatus have especial utility in surgical and diagnostic interventions, including such interventions involving the central nervous system encased in surrounding skull or the like.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims the benefit of U.S. application Ser. No. 09/514,008, filed Feb. 25, 2000 now abandoned which claims the benefit of Provisional application No. 60/122,484 filed Feb. 26, 1999. 
    
    
     FIELD OF THE INVENTION 
     This invention pertains to instrument systems and methods for positioning the body, or a portion of the body, of a surgical subject (or other “body” as defined herein) at a predetermined three-dimensional position in space. The systems and methods have especial utility for surgery, diagnostic intervention, and research involving the subject&#39;s brain or other anatomical structure located in the interior of the subject&#39;s body, wherein the brain or other anatomical structure has a buried locus of interest that normally is obscured by overlying structure. 
     BACKGROUND OF THE INVENTION 
     In research and surgery of animals including small animals such as rats and mice, it can be extremely difficult to locate a terminus of a probe, electrode, micropipette, or other implement (herein generally termed a “probe”) at a particular location within the subject&#39;s body without having to remove overlying structure and the like to permit direct observation of placement of the probe. This problem is especially critical in brain research involving the placement of a probe at a desired locus deep within a living subject&#39;s brain inside the surrounding skull. 
     To aid researchers in locating various anatomical structures in the brains of research animals such as mice, rats, cats, dogs, and primates, respective so-called brain atlases are often consulted. A brain atlas provides three-dimensional coordinates for the structures, normally using a Cartesian (rectangular) coordinate system, relative to one or more accessible anatomical features. (For example, for mice and rats, the usual reference feature on the skull is bregma, which is a point of meeting of the coronal and sagittal sutures. A second reference feature that is sometimes used in connection with bregma is lambda, which is located posteriorly of bregma and is a point of meeting of the lambdoidal and sagittal sutures. The sagittal suture connecting bregma and lambda is regarded generally as representing a sagittal mid-line of the skull.) However, despite the existence of such information, current apparatus and methods used to place an introduced probe are notoriously inaccurate with individual subjects and from one subject to another in a population of subjects. Such inaccuracy is a substantial problem because it results in unintentionally mis-positioned probes and other tools, which causes misleading research data and wasted animal resources. 
     Stereotaxic apparatus are known in the art for positioning a subject&#39;s head for brain research. For a small animal such as a mouse or rat, the head is held immobile by externally applied structures such as ear bars and a nose clamp providing a “three-point” holding system. As an example, reference is made to U.S. Pat. No. 5,601,570 to Altmann et al. 
     All known prior-art apparatus have various substantial shortcomings. For example, the Altmann et al. apparatus is inherently incapable of positioning a subject&#39;s head, in three-dimensional space, in a manner providing a high level of confidence that a probe inserted from outside the skull will “hit” a desired locus within the brain. More specifically, the Altmann et al. apparatus does not allow the researcher, intending to probe a living brain of a research animal, to position a particular animal&#39;s head in a manner providing reliably accurate insertion and placement of the probe to desired three-dimensional coordinates in the brain. The Altmann apparatus also exhibits poor precision of placements of a probe at a desired locus in each animal in a population of animals. Consequently, the researcher must conduct a series of “pilot” studies, followed by histological confirmations, to compare actual probe results with desired results (e.g., to compare actual hit loci with desired hit loci based on information in a brain atlas). Such studies using conventional apparatus usually produce data exhibiting wide variations that often are attributed wrongly to biological variations among individual animals in a population, strains, ages of animals, and so on. As the pilot studies progress, the coordinates provided by a conventional apparatus are adjusted gradually to compensate for the variation and to improve the hit rate. Unfortunately, such studies are time-consuming and costly to perform, and require substantially increased numbers of animals to conduct a particular experiment. Conventional instruments simply do not allow the researcher to differentiate between the many sources of error. Furthermore, even with adjustments to the apparatus based on the pilot studies, hit rates remain disappointingly low, resulting in inconclusive research. 
     As noted above, individual animals (even of the same strain) exhibit substantial variation, one animal to the next, in morphology of body structures such as the skull. If positioning of the body or body structure is guided, according to the prior art, solely on the basis of external features (e.g., positions of ear holes relative to each other and to the snout), this variation usually results in excessive variation in probe placement at target loci within the brain. 
     SUMMARY OF THE INVENTION 
     In view of the shortcomings of the prior art summarized above, the present invention provides, inter alia, apparatus and methods for positioning the body, or portion of the body (such as the skull and its contents), of a research subject accurately in three-dimensional space. (As used herein, the term “body” can be an entire body such as an entire mouse or rat, or a portion of an entire body.) To achieve such positioning, the body is held in a holder configured to hold the body immobile in a desired position. The holder, in turn, is mounted in a manner allowing any of various motions in three-dimensional space required to achieve the desired positioning. 
     According to a first aspect of the invention, stereotaxic holders are provided for holding a body at a position in three-dimensional space. A representative embodiment of such a holder comprises a frame, an X-axis shift mechanism, a Y-axis shift mechanism, and a Z-axis shift mechanism (wherein the terms “X-axis,” “Y-axis,” and “Z-axis” refer to the orthogonal axes in a Cartesian coordinate system. A body-holding component, configured to contact a body, can be attached to the frame such that the body-holding component extends from the frame to contact the body and hold the body relative to the frame. The frame is attached to the X-axis, Y-axis, and Z-axis shift mechanisms. The X-axis shift mechanism is configured to move the frame, with body-holding component, along an X-axis. The Y-axis shift mechanism is configured to move the frame, with body-holding component, along a Y-axis, wherein the movement along the Y-axis is independent of the movement along the X-axis. The Z-axis shift mechanism is configured to move the frame, with body-holding component, along a Z-axis, wherein the movement along the Z-axis is independent of the movement along the X-axis or along the Y-axis. The shift mechanisms are configured relative to each other so as to define a reference X-axis, a reference Y-axis, and a reference Z-axis, respectively, that are orthogonal relative to each other and that mutually intersect at a 0,0,0 point in three-dimensional space. The X-axis shift mechanism, Y-axis shift mechanism, and Z-axis shift mechanism are configured to move a body, mounted to the frame by the body-holding component, as required to place a selected point on or in the body at the 0,0,0 point. 
     The stereotaxic holder as summarized above can further comprise one or more of an X-axis tilting mechanism, a Y-axis tilting mechanism, and a Z-axis tilting mechanism. The X-axis tilting mechanism is configured to tilt a body, held by the frame, about the reference X-axis and relative to the 0,0,0 point. The Y-axis tilting mechanism is configured to tilt a body, held by the frame, about the reference Y-axis and relative to the 0,0,0 point. The Z-axis tilting mechanism is configured to tilt a body, held by the frame, about the reference Z-axis and relative to the 0,0,0 point. Each tilting motion is independent of any other tilting motion of the body or of any shifting motion of the frame as achieved by the stereotaxic holder. 
     The stereotaxic holder can further comprise at least one body-holding component attached to the frame. Exemplary body-holding components include, but are not limited to, ear bars and snout adapters. 
     In an example embodiment of a stereotaxic holder according to the invention, the frame is attached to the Z-axis shifting mechanism, the Z-axis shifting mechanism is attached to the X-axis shifting mechanism, and the X-axis shifting mechanism is attached to the Y-axis shifting mechanism. The example embodiment can further comprise a plate, wherein the X-axis tilting mechanism is attached to the plate. Hence, the Y-axis shifting mechanism is attached to the X-axis tilting mechanism, the Y-axis tilting mechanism is attached to the Y-axis shifting mechanism, the X-axis shifting mechanism is attached to the Y-axis tilting mechanism, and the Z-axis shifting mechanism is attached to the X-axis shifting mechanism. The plate can be mounted pivotably to a sub-plate to allow the plate to swing about the reference Z-axis. In such a configuration, the plate and sub-plate comprise the Z-axis tilting mechanism. 
     A second representative embodiment of a stereotaxic holder according to the invention comprises a first U-frame, a Z-axis shifting mechanism, an X-axis shifting mechanism, a Y-axis shifting mechanism, a Y-axis tilting mechanism, an X-axis tilting mechanism, and a Z-axis swing mechanism. A body-holding component, as summarized above, is attached to the first U-frame. The first U-frame is attached to the Z-axis shifting mechanism, which is configured to move the first U-frame, with body-holding component, along a Z-axis. The Z-axis shifting mechanism is attached to the X-axis shifting mechanism, which is configured to move the Z-axis shifting mechanism and first U-frame along an X-axis. The X-axis shifting mechanism is attached to the Y-axis shifting mechanism, which is configured to move the X-axis shifting mechanism, Z-axis shifting mechanism, and first U-frame along a Y-axis. The Y-axis tilting mechanism connects the X-axis shifting mechanism to the Y-axis shifting mechanism. The Y-axis tilting mechanism defines a reference Y-axis about which the Y-axis tilting mechanism effects tilting of the body. The Y-axis tilting mechanism is attached to the X-axis tilting mechanism, and the X-axis tilting mechanism is attached to the Z-axis swing mechanism. The X-axis tilting mechanism defines a reference X-axis about which the X-axis tilting mechanism effects tilting of the body, and the Z-axis swing mechanism defines a reference Z-axis about which the Z-axis swing mechanism effects a swing of the body. The reference X-axis, reference Y-axis, and reference Z-axis are orthogonal to each other and mutually intersect at a 0,0,0 point in three-dimensional space. 
     In the second representative embodiment as summarized above, the X-axis tilting mechanism can comprise a second U-frame having ends that pivot about the reference X-axis, and a base to which the Y-axis shifting mechanism is attached. In such a configuration, the Z-axis swing mechanism can comprise a plate and a sub-plate, wherein the X-axis tilting mechanism is attached to the plate and the plate is attached pivotably to the sub-plate to allow the plate to swing about the reference Z-axis. 
     According to another aspect of the invention, stereotaxic alignment systems are provided. A representative embodiment of such a system comprises a base plate and any of various stereotaxic holders according to the invention. For example, the stereotaxic holder can be configured as summarized above with respect to the first representative embodiment. In such a configuration, the stereotaxic holder can further comprise at least one of (desirably all three of) an X-axis tilting mechanism, a Y-axis tilting mechanism, and a Z-axis tilting mechanism. Each tilting mechanism, if present, is configured to tilt a body, held by the frame, about the respective reference axis and relative to the 0,0,0 point independently of any other tilting motion of the body or of any shifting motion of the frame. 
     In a stereotaxic alignment system according to the invention, the stereotaxic holder can include a centering gauge indicating the 0,0,0 point. For example, the centering gauge can be situated on the terminal face of a gauge post attached to the stereotaxic holder such that the gauge post is coaxial with the reference Z-axis. 
     Another representative embodiment of a stereotaxic alignment system according to the invention comprises a base plate, a stereotaxic holder (as summarized above) mounted to the base plate, and a manipulator mounted to the base plate. The manipulator includes a “controlled end” to which an implement can be mounted. Thus, the manipulator can present to the body a tool, held by the manipulator, at a desired locus on or in the body relative to the 0,0,0 point. 
     The manipulator desirably comprises an X-axis shifting mechanism, a Y-axis shifting mechanism, and a Z-axis shifting mechanism for shifting the controlled end along an X-axis, Y-axis, and Z-axis, respectively, relative to the 0,0,0 point. The manipulator further comprises a three-axis universal joint to which the X-axis shifting mechanism, the Y-axis shifting mechanism, and Z-axis shifting mechanism are mounted. The universal joint desirably is configured to allow adjustment of an orthogonal relationship of the X-axis, Y-axis, and Z-axis of the manipulator relative to each other. The universal joint can be configured further to allow adjustment of one or more of the X-axis, Y-axis, and Z-axis of the manipulator with one or more of the reference X-axis, reference Y-axis, and reference Z-axis of the stereotaxic holder. 
     In a stereotaxic alignment system according to the invention, the manipulator can include an implement mounted to the controlled end of the manipulator. Desirably, any of various implements has an alignment axis (usually the longitudinal axis of the implement). Desirably, any implement attachable to the controlled end is “self-indexing” as defined herein. 
     An exemplary implement is a centering scope usable with a centering gauge, as summarized above, that indicates the 0,0,0 point. The centering scope has an optical axis that is coincident with the alignment axis. In such an arrangement, the manipulator is configured to position the centering scope in an adjustable manner such that the optical axis intersects the centering gauge at the 0,0,0 point. 
     Other exemplary implements include, but are not limited to, drilling units, syringe holders, dial test indicators, cannula-insertion devices, and a stereotaxic alignment indicators. 
     According to another aspect of the invention, methods are provided for performing a stereotaxic alignment of a body. According to a representative embodiment of such a method, a reference X-axis, a reference Y-axis, and a reference Z-axis are provided that are orthogonal to each other and that mutually intersect at a 0,0,0 point in three-dimensional space. The body is mounted in a holder configured to effect respective controlled shifts of the body in an X-axis direction, a Y-axis direction, and a Z-axis direction, and to effect respective controlled tilts of the body about the reference X-axis and reference Y-axis, as well as controlled swings of the body about the reference Z-axis. Using the holder, the body is shifted as required in the X-axis, Y-axis, and Z-axis dimensions to place a selected target point on or in the body at the 0,0,0 point. Further using the holder, the body is subjected to a swinging motion as required about the reference Z-axis while maintaining the target point at the 0,0,0 point, to obtain a desired orientation of the body relative to the reference Y-axis or the reference X-axis. Further using the holder, the body is tilted as required about the reference Y-axis while maintaining the target point at the 0,0,0 point, so as to obtain a desired orientation of the body relative to the reference X-axis. Further using the holder, the body is tilted as required about the reference X-axis while maintaining the target point at the 0,0,0 point, so as to obtain a desired orientation of the body relative to the reference Y-axis. The step of swinging the body about the reference Z-axis can comprise the steps of: (1) identifying a first reference point and a second reference point on or in the body, wherein the first and second reference points define a reference line; and (2) swinging the body as required about the reference Z-axis until the reference line is at a desired orientation relative to the reference X-axis or the reference Y-axis. The reference line can be, for example, a sagittal axis of the body, wherein placing the reference line at the desired orientation achieves a sagittal alignment of the body. 
     The step of tilting the body about the reference Y-axis can comprise the steps of: (1) providing a stereotaxic alignment indicator for ascertaining the orientation of the body relative to the reference X-axis; (2) placing the stereotaxic alignment indicator into functional contact with the body; and (3) tilting the body as required until the stereotaxic alignment indicator indicates the desired orientation of the body about the reference Y-axis relative to the reference X-axis. For example, the body can be aligned to have its sagittal axis aligned with the reference Y-axis, wherein obtaining the desired orientation of the body about the reference Y-axis places the body at a desired coronal tilt. 
     The step of tilting the body about the reference X-axis can comprise the steps of: (1) providing a stereotaxic alignment indicator for ascertaining the orientation of the body relative to the reference Y-axis; (2) placing the stereotaxic alignment indicator into functional contact with the body; and (3) tilting the body as required until the stereotaxic alignment indicator indicates the desired orientation of the body about the reference X-axis relative to the reference Y-axis. For example, the body can be aligned to have its sagittal axis aligned with the reference Y-axis, wherein obtaining the desired orientation of the body about the reference X-axis places the body at a desired dorsal tilt. 
    
    
     The foregoing and additional features and advantages of the invention will be more apparent from the following detailed description, which proceeds with reference to the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG.  1 ( a ) is an oblique rear view of a representative stereotaxic holder, according to the invention, including a snout adapter and ear bars for holding a rodent skull. 
     FIG.  1 ( b ) is an oblique front view of the FIG.  1 ( a ) embodiment, including a snout adapter and ear bars for holding a rodent skull. 
     FIG.  1 ( c ) is an oblique front view of the FIG.  1 ( a ) embodiment, but with the snout adapter and ear bars removed and a gauge post attached. 
     FIG. 2 is an oblique front view of the stereotaxic holder embodiment of FIG.  1 ( a ) attached to a base. 
     FIG. 3 is an oblique front view of a representative embodiment of a stereotaxic alignment system according to the invention, the system including the FIG. 2 embodiment including a manipulator attached to the base and a centering scope attached to the manipulator. 
     FIG. 4 is an enlarged oblique front view of a representative embodiment of a manipulator that can be included with a stereotaxic alignment system according to the invention. The FIG. 4 manipulator is substantially the same as shown in FIG.  3 . 
     FIG. 5 shows details of a centering scope as a first representative implement that can be mounted to a manipulator of a stereotaxic alignment system according to the invention. 
     FIG. 6 shows details of a drilling unit as a second representative implement that can be mounted to a manipulator of a stereotaxic alignment system according to the invention. 
     FIG. 7 shows details of a syringe holder as a third representative implement that can be mounted to a manipulator of a stereotaxic alignment system according to the invention. 
     FIG. 8 shows details of a dial test indicator as a fourth representative implement that can be mounted to a manipulator of a stereotaxic alignment system according to the invention. 
     FIG. 9 shows details of a first representative embodiment of a cannula-insertion device as yet another example implement that can be mounted to a manipulator of a stereotaxic alignment system according to the invention. 
     FIG. 10 shows details of a second representative embodiment of a cannula-insertion device as yet another example implement that can be mounted to a manipulator of a stereotaxic alignment system according to the invention. 
     FIG. 11 shows details of a third representative embodiment of a cannula-insertion device as yet another example implement that can be mounted to a manipulator of a stereotaxic alignment system according to the invention. 
     FIG. 12 depicts a cannula adapter, for use in the FIG. 11 embodiment of a cannula-insertion device, placed in a gauge block  900  used for facilitating auto-indexing of a cannula or other tool held by the cannula adapter when mounted to the cannula-insertion device. 
     FIG. 13 shows details of a representative embodiment of a stereotaxic alignment indicator as yet another example implement that can be mounted to a manipulator of a stereotaxic alignment system according to the invention. 
     FIG.  14 ( a ) is a front oblique view similar to FIG. 3, but in which a stereotaxic alignment indicator (such as shown in FIG. 13) is mounted to the manipulator rather than the centering scope. The stereotaxic manipulator is oriented to perform a determination of tilt of the subject body about a Y-axis. 
     FIG. 14 is a front oblique view similar to FIG.  14 ( a ), but in which the stereotaxic alignment indicator is oriented to perform a determination of tilt of the subject body about an X-axis. 
     FIG. 15 is a front oblique view of a representative embodiment of a snout adapter that can be mounted to a stereotaxic holder according to the invention. 
    
    
     DETAILED DESCRIPTION 
     To better understand the various motions of a body achievable using an apparatus according to the invention, the following information is useful. (When reviewing this information, it is helpful to envision a human body standing on its feet and facing straight ahead.) A median plane is a vertical plane that divides the body lengthwise into right and left halves. This plane is also termed a sagittal plane (because in a standing human it passes approximately through the sagittal suture in the skull), but actually any plane parallel to the medial plane is also termed a sagittal plane. A sagittal line or axis is a line on the sagittal plane extending lengthwise with respect to the subject&#39;s body (with a skull, such a line would extend roughly parallel to at least a portion of the sagittal suture). The coronal plane is a vertical plane that is perpendicular to the sagittal plane. (The coronal plane is so termed because in a standing human it passes approximately through the coronal suture in the skull.) Thus, the coronal plane divides the body into a front (ventral) half and a rear (dorsal) half. A coronal line or axis is a line in the coronal plane extending widthwise with respect to the subject&#39;s body (with a skull, such a line would extend roughly parallel to at least a portion of the coronal suture). A transverse plane is perpendicular to the sagittal and coronal planes. Ventral refers to the front (or belly surface) of the subject, and dorsal refers to the rear (or back surface) of the subject. Ventral and dorsal are synonymous with anterior and posterior, respectively. 
     A representative embodiment of a stereotaxic alignment system according to the invention comprises a stereotaxic holder  10  (such as the embodiment shown in FIGS.  1 ( a ) and  1 ( b ) and described below). For improved stability, the stereotaxic holder  10  desirably is mounted on a heavy base such as shown in FIG.  2 . 
     The embodiment of FIGS.  1 ( a ) and  1 ( b ) is adapted especially for mounting and positioning of the head of a surgical or diagnostic subject (e.g., a rodent) in three-dimensional space to a reference position, and for rotating the subject&#39;s head into a desired three-dimensional position or stereotaxic plane. To such end, the stereotaxic holder comprises multiple slide mechanisms for controlled movement and placement of the subject&#39;s head in all three Cartesian dimensions. However, it readily will be appreciated that the relative dimensions of components of this embodiment can be changed to enable the apparatus to accommodate any size or configuration of body or body structure to be held by it. 
     A subject “body” (which can be a portion of an actual body) is held using components conveniently mounted to a first U-frame  12 . The first U-frame  12  includes a center portion (base)  14  and first and second arms  16 ,  18 , respectively. As discussed later, to the center portion  14  can be attached, for example, an appropriate snout adapter for holding the anterior end of a rodent skull. Each arm  16 ,  18  of the first U-frame  12  terminates with a respective ear bar  20 ,  22 , respectively. The ear bars  20 ,  22  extend from the respective arms  16 ,  18  toward each other to engage the respective ear openings in the subject&#39;s skull. For ease of mounting the skull to the first U-frame  12 , the spacing between the ear bars  20 ,  22  desirably is adjustable by loosening knurled screws  24 ,  26 , sliding the ear bars  20 ,  22  toward or away from each other (a scale  25  on each ear bar  20 ,  22  can be used as a guide), and then tightening the knurled screws  24 ,  26 . 
     It readily will be appreciated that the ear bars  20 ,  22  can be replaced with any of various other grasping, centering, or holding implements especially configured to engage a particular corresponding physical feature of a subject body to be held by the stereotaxic holder  10 . 
     The first U-frame  12  is supported by an assembly of slide (or “shift”) mechanisms that collectively allow positioning motions of the first U-frame  12  (and thus a skull or other body held by the first U-frame) linearly along the three Cartesian axes (X-, Y-, and Z-axis). Briefly, referring to FIG.  1 ( a ), a Z-axis shift mechanism  28  provides shift motion of the first U-frame  12  along the indicated Z-axis; an X-axis shift mechanism  30  provides shift motion of the first U-frame  12  along the indicated X-axis; and a Y-axis shift mechanism  32  provides shift motion of the first U-frame  12  along the indicated Y-axis. Motion along the Z-axis is achieved by turning the knurled knob  34  that actuates the Z-axis shift mechanism  28 . Motion along the X-axis is achieved by turning the knurled knob  36  that actuates the X-axis shift mechanism  30 . Motion along the Y-axis is achieved by turning the knurled knob  38  that actuates the Y-axis shift mechanism  32 . 
     Whereas, in the embodiment of FIGS.  1 ( a ) and  1 ( b ), the knurled knobs  34 ,  36 ,  38  are adapted especially for manual turning, it is contemplated that such turnings can be made using, by way of example, any of various wheels, cranks, levers, or motors. Also, whereas the depicted embodiment utilizes slide mechanisms each employing parallel guide bars and a lead screw, as described below, it readily will be apparent that any of various other linear displacement mechanisms can be employed, such as (but not limited to) dovetail slides and linear ball slides. 
     In more detail, the base  14  of the first U-frame  12  is attached to the Z-axis shift mechanism  28 . The Z-axis shift mechanism  28  comprises two parallel bushings  42 ,  44  or linear bearings (generally termed “bushings”) mounted in the base  14  of the first U-frame  12 . Respective parallel guide bars  46 ,  48  are inserted into the bushings  42 ,  44  and extend to respective arms  51 ,  53  of a first T-member  50  to which the guide bars are affixed. A lead screw  52 , attached to the knurled knob  34 , extends through the first T-member  50 , and is threaded into the base  14  of the first U-frame  12 . Thus, turning the knurled knob  34  causes shift motion (along the Z-axis) of the first U-frame  12  along the guide bars  46 ,  48  relative to the first T-member  50 . 
     Turning now to the X-axis shift mechanism  30 , two parallel bushings  54 ,  56  or linear bearings (generally termed “bushings”) are mounted in the stem  58  of the first T-member  50 . Respective parallel guide bars  60 ,  62  are inserted into the bushings  54 ,  56  and extend to opposing arms  64 ,  66  of a second U-frame  68  to which the guide bars  60 ,  62  are affixed. A lead screw  70  is attached to the knurled knob  36 , extends through one arm  64  of the second U-frame  68 , is threaded into the stem  58  of the first T-member  50 , and is journaled in the second arm  66  of the second U-frame  68 . Thus, turning the knurled knob  36  causes shift motion (along the X-axis) of the first T-member  50  (with attached first U-frame  12 ) along the guide bars  60 ,  62  relative to the second U-frame  68 . 
     Turning now to the Y-axis shift mechanism  32 , two parallel bushings  72 ,  74  or linear bearings (generally termed “bushings”) are mounted in the base  90  of a third U-frame  76 . Respective parallel guide bars  78 ,  80  are inserted into the bushings  72 ,  74  and extend to respective arms  82 ,  84  of a second T-member  86  to which the guide bars  78 ,  80  are affixed. A lead screw  88  is attached to the knurled knob  38 , extends through the second T-member  86 , and is threaded into the base  90  of the third U-frame  76 . Thus, turning the knurled knob  38  causes shift motion (along the Y-axis) of the X-axis shift mechanism  30 , the Z-axis shift mechanism  32 , and the first U-frame  12  along the guide bars  78 ,  80  relative to the third U-frame  76 . 
     In addition to the X-, Y-, and Z-axis shift mechanisms  30 ,  32 ,  28 , respectively, discussed above for achieving respective linear positioning motions along the three Cartesian axes, the embodiment of FIGS.  1 ( a ) and  1 ( b ) also is configured to effect pivoting (“tilt” and “swing”) motions about each of three Cartesian reference axes. 
     The reference Y-axis about which Y-axis tilting motion can be achieved is denoted “PAX Y ” in FIGS.  1 ( a ) and  1 ( b ) and extends through the stem  92  of the second T-member  86 . Specifically, a shaft  94  is attached to the base  96  of the second U-frame  68  and is journaled in the stem  92  of the second T-member  86 . A knurled knob  98  is used to effect rotation of a gear (or analogous means) engaged with the shaft  94  or with the base  96 ; i.e., turning of the knurled knob  98  effects tilting of the second U-frame  68 , Z-axis shift mechanism  28 , and X-axis shift mechanism  30  (and all components attached thereto) about the axis PAX Y . A particular angular position about the axis PAX Y  can be “locked” by tightening a cinching screw  100 . 
     The reference X-axis about which X-axis tilting motion can be achieved is denoted “PAX X ” in FIGS.  1 ( a ) and  1 ( b ). The axis PAX X  extends through the termini of the arms  102 ,  104  of the third U-frame  76 . Specifically, the arms  102 ,  104  are attached via respective shafts  106 ,  108  to respective blocks  110 ,  112  allowing tilting motion of the third U-frame  76  (including all components attached thereto) about the axis PAX X . To achieve such tilting motion in a controllable manner, a respective block or other suitable member  114 ,  116  is attached rotatably to each arm  102 ,  104  of the third U-frame  76 . (The figure shows such attachment at about the midline of each arm  102 ,  104 , but such a configuration is not limiting in any way.) Threaded through one block  114  is a lead screw  118  terminating with a knurled knob  120 , and extending through the other block  116  is a guide bar  122 . The lead screw  118  is affixed rotatably to a respective member  124  that, in turn, is journaled in a respective block  126  or other suitable member. Similarly, the guide bar  122  is affixed to a respective member  128  that, in turn, is journaled in a respective block  130  or other suitable member. The blocks  126 ,  130  are affixed to a plate  132  to which the blocks  110 ,  112  are also attached. Thus, turning the knurled knob  120  effects tilting motion of the third U-frame  76  relative to the plate  132  about the axis PAX X . After attaining a desired position of the third U-frame  76 , a cinching screw  131  can be tightened onto the guide bar  122 . 
     The reference Z-axis about which Z-axis pivoting motion (“swing”) can be achieved is denoted “PAX Z ” in FIGS.  1 ( a ) and  1 ( b ). The axis PAX z  extends through the plate  132  into a sub-plate  134 . The plate  132  is attached rotatably to the sub-plate  134  in any suitable manner allowing motion of the plate  132  relative to the sub-plate  134  about the axis PAX Z . Motion of this type in a controlled manner desirably is effected by turning a shaft  136  engaged (e.g., by a gear engagement or tire engagement) with a curved edge  138  of the plate  132 . The shaft  136  is journaled in the sub-plate  134  and terminates with a knurled knob  140 . Thus, turning the knurled knob  140  effects motion (swing) of the plate  132  about the axis PAX Z  relative to the sub-plate  134 . The angular orientation of the plate  132  relative to the sub-plate  134  can be ascertained by consulting a protractor scale  142 . The desired angular orientation can be “locked” by tightening a cinching screw  144 . 
     FIG.  1 ( c ) shows, extending upward along the axis PAX Z , a removable gauge post  146  terminating with a “centering gauge”  148 . The gauge post  146  has a fixed length relative to the plate  132  and is adapted to be mounted on a pad  150  on the plate  132 . Whenever the gauge post  146  is so mounted, it is coaxial with the axis PAX Z , and the axes PAX X  and PAX Y  pass through and intersect in the center of the centering gauge  148 . The centering gauge  148  includes an appropriate cross-hair reticle or target that indicates the point of intersection of the three axes PAX X , PAX Y , and PAX Z . During operation of the FIG. 1 embodiment, all axes of rotation for aligning the subject&#39;s skull (or other body held by the first U-frame  12 ) into the desired stereotaxic plane are focused at this mutual point of intersection. For example, if the FIG. 1 apparatus is used to hold a mouse head, then the point of intersection can be at bregma on the subject skull to establish a center of rotation, at bregma, for all three axes PAX X , PAX Y , PAX Z . The ability of an apparatus according to the invention to establish this focal point of rotation for all three axes is in stark contrast to apparatus according to the prior art in which the focus of rotation is centered about, e.g., the intersection of “ear bar zero” and the medial plane. With a rodent skull, use of bregma as the center of rotation for all three axes is a key advantage in being able, using an apparatus according to the invention, to position a probe very accurately at a locus within the rodent brain that is situated, according to a brain atlas or other reference data, at a specified location relative to bregma or relative to bregma and lambda. 
     It will be appreciated readily that the principles of the present invention are not limited to centering on bregma. Rather, any natural or artificial reference point on or in a body can be used. For example, aside from any of various natural anatomical reference points, the reference point actually utilized can be an implanted bead of a substance readily visible using an X-ray, ultrasonic imager, or MRI imager. Furthermore, although the components of any of the various embodiments within the scope of the invention desirably are made of metal (e.g., aluminum alloy) for most applications, some or all the components can be made of any of various other suitable rigid materials. For example, the stereotaxic holder  10  can be made of a rigid polymer that enables the holder to be used with an MRI imager without the stereotaxic holder itself interfering with MRI imaging of the body being held by the stereotaxic holder. 
     Depending upon the orientation of the body mounted to the apparatus of FIGS.  1 ( a ) and  1 ( b ), the axis PAX Z  can be regarded as a “sagittal-swing axis,” wherein a swing about the axis PAX Z  is made as required to achieve sagittal alignment of the subject body. In the case of a rodent skull, sagittal alignment can achieve alignment of the sagittal suture with (parallel to) anterior-posterior motions of a manipulator (described below) usable in conjunction with the embodiment of FIGS.  1 ( a ) and  1 ( b ), wherein bregma and lambda are used as reference points for the alignment. I.e., sagittal alignment under such conditions results in bregma and lambda defining a line extending perpendicularly to the axis PAX X  passing through bregma. Under such conditions, the axis PAX X  can be regarded as a “dorsal-tilt axis” for aligning (in the context of a rodent skull) the sagittal suture exactly perpendicularly (or at another pre-determined angular orientation) relative to vertical motions of the manipulator. In other words, nose-up or nose-down tilts are made as required to align lambda with bregma horizontally (with the axis of rotation, PAX X , passing horizontally through bregma and being parallel with lateral motions of the manipulator). Finally, under such conditions, the axis PAX Y  can be regarded as a “coronal-tilt axis” about which the subject skull can be tilted laterally. The coronal-tilt axis passes horizontally through bregma and lambda parallel to the anterior-posterior motions of the manipulator. 
     Referring again to the embodiment shown in FIG.  1 ( c ), first and second mounting bars  152 ,  154  extend from the base  14  of the first U-frame  12 . The mounting bars  152 ,  154  extend toward the gauge post  146  and can be used for mounting an appropriate snout adapter  160  (FIG.  1 ( b )) or other suitable holding implement for the particular body to be held by the stereotaxic holder  10 . A representative snout adapter is described later below. 
     Referring now to FIG. 2, a representative embodiment of a base  180  comprises a base plate  182  having first and second opposing lateral edges  184 ,  186 . Adjacent and coextensive with each lateral edge is a respective dovetail rail  188 ,  190  or alternative analogous slide mechanism allowing attachment and detachment of implements (such as a manipulator  200  as described below) as well as controlled movement of attached implement(s) in the directions in which the rails  188 ,  190  extend. Beneath each corner of the base plate is a respective non-slip, adjustable, leveling pad  192  used to keep the base  180  level on a working surface and to keep the base firmly in place on the working surface. The base  180  shown in FIG. 2 includes a stereotaxic holder  10 , such as the embodiment shown in FIGS.  1 ( a )- 1 ( c ), mounted thereto. 
     A representative embodiment of a manipulator  200  is shown in FIGS. 3 and 4. The depicted embodiment comprises a dovetail slide block  202  adapted to be mounted onto a dovetail rail  188 ,  190  (shown mounted on the dovetail rail  188 ) of the base  180 , thereby permitting alignment of the Cartesian axes (X-, Y-, and Z-axes) of the manipulator  200  with the Cartesian axes PAX X , PAX Y , PAX Z  of the stereotaxic holder  10 . After mounting the manipulator  200  to the desired rail and sliding it to the desired location on the rail, the manipulator  200  can be affixed rigidly to the rail by tightening a cinching screw  204  (obstructed by foreground structure in this view) threaded into the slide block  202 . The manipulator  200  comprises a distal “controlled end”  206  and a first shift mechanism  208 , a second shift mechanism  210 , and a third shift mechanism  212  for achieving controlled shift motions of the controlled end  206  along each of the three Cartesian axes (i.e., along the Y-axis, Z-axis, and X-axis, respectively). The first shift mechanism  208  is used for moving (after the manipulator  200  is mounted to the base  180 ) the controlled end  206  in the Y-direction in a controlled manner. To such end, the knurled knob  214  is turned, which causes a corresponding shift movement of a block  207  in the Y-direction relative to the block  202  (FIG.  4 ). The second shift mechanism  210  is used for moving the controlled end  206  in the Z-direction in a controlled manner. To such end, the knurled knob  216  is turned, which causes a corresponding shift movement of a block  209  relative to a guide member  211 . Finally, the third shift mechanism  212  is used for moving the controlled end  206  in the X-direction in a controlled manner. To such end, the knurled knob  218  is turned, which causes a corresponding shift movement of a guide member  213  relative to the block  209 . Each shift mechanism  208 ,  210 ,  212  is configured in the illustrated embodiment as a dovetail slide mechanism. However, as discussed above with respect to the stereotaxic holder  10 , any of the shift mechanisms  208 ,  210 ,  212  alternatively can be any of various other analogous mechanisms. In the illustrated embodiment, each knurled knob  214 ,  216 ,  218  is attached to the terminus of a respective threaded shaft (not visible in the drawing). 
     In the embodiment depicted in FIGS. 3 and 4, each shift mechanism  208 ,  210 ,  212  includes a respective electronic digital scale  220 ,  222 ,  224  that displays a measured position along the respective axis. (In a representative alternative embodiment, respective vernier scales, rather than electronic digital scales, can be used to display shift position along each of the respective Cartesian axes.) Compared to a vernier scale, an electronic digital scale has advantages including greater resolution, lesser probability of reading errors, and capability of being reset to “zero” along the respective axis. Exemplary digital scales include DIGIMATIC™ scales (e.g., series  572 ) manufactured by Mitutoyo, Japan. Another candidate digital scale is any of various highly accurate “glass scales” such as DRO model  211  manufactured by Anilam, Miramar, Fla. By way of example only, with respect to a manipulator having shift movement ranges suitable for a mouse or rat animal subject, each of the shift mechanisms  208 ,  210 ,  212  has a motion range of 70 mm along the respective axis. It will be understood readily that these ranges can be made larger or smaller as required to accommodate larger or smaller subjects, respectively. 
     The controlled end  206  of the manipulator  200  is configured (by any of various possible attachment means) to have any of various implements attached to it. Thus, after performing alignment of the Cartesian axes of the manipulator  200  with the respective axes of the stereotaxic holder  10 , an attached implement can be shifted along each of the Cartesian axes of the stereotaxic holder  10  in a controlled manner. By way of example, the depicted embodiment (FIG. 4) defines a female dovetail block  226 . Each of the various implements that are attachable individually to a controlled end  206  having such a configuration has a conforming male dovetail rail segment mounted to an adapter block. The male dovetail rail segment allows the implement to slide into the female dovetail block  226  and thus be affixed to the controlled end  206 . A cinching screw  228  is used to tighten the implement on the controlled end  206 . 
     Desirably, for reasons that will be more apparent from the following discussion, the adapter block on each implement desirably is “self-indexing” with respect to the controlled end  206 . By “self-indexing” is meant that any of various implements attachable to the controlled end can be attached with the functional end of the implement being at the same location, in three dimensional space, from one implement to the next. To such end, using the depicted embodiment by way of example, the female dovetail block  226  on the manipulator and/or the adapter block on each implement is provided with a mechanical stop (e.g., a pin or the like, not shown) that engages the other block in a consistent manner. Thus, the adapter block of any of various implements is mountable at exactly the same position, from one implement to another, relative to the female dovetail block  226 . Self-indexing allows any of various implements to be attached to the manipulator without a need to re-adjust the manipulator or implement immediately after each mounting. 
     Many implements mountable to the controlled end  206  have a longitudinal axis O Z . Another advantage of the “self-indexing” feature is that the axis O Z  of any implement mounted on the controlled end  206  is, so long as the manipulator has not been adjusted in the meantime, automatically coincident with the axis O Z  of the previous implement and/or the subsequent implement mounted to the controlled end  206 . Again, this eliminates a need to re-adjust the manipulator  200  after changing the implement mounted to the controlled end  206 . 
     The manipulator  200  desirably also includes a 3-axis universal joint  230 . As shown in FIG. 4, the universal joint  230  comprises a first pivot block  232  mounted to an end of the block  207 , a second pivot block  234  tiltably mounted to the first pivot block  232 , and a third pivot block  236  swingably mounted to the second pivot block  234 . An end of the guide member  211  is mounted to the third pivot block  236 . The first pivot block  232  is tilted controllably as required about a first pivot axis P Y  relative to the block  207  by turning a respective jack screw  238 . The second pivot block  234  is tilted controllably about a second pivot axis P X  relative to the first pivot block  232  by turning a respective jack screw  240 . The third pivot block  236  is swung controllably as required about a third pivot axis P Z  relative to the second pivot block  234  by turning a respective jack screw  242 . Such controlled tilt and swing motions about one or more of the respective axes P X , P Y , and P Z  are normally extremely limited in scope. They are performed normally whenever it is desired or necessary to bring the three Cartesian axes of shift motion of the controlled end  206  (achieved by the manipulator  200 ) into exact orthogonal relationship with each other and/or to align the three Cartesian axes of shift motion of the controlled end  206  (achieved by the manipulator  200 ) exactly with the three Cartesian axes of the stereotaxic holder  10 . Such adjustments can be advantageous after the manipulator  200  and/or stereotaxic holder  10  are mounted to the base  180 . 
     A first example implement mountable to the controlled end  206  is a centering scope  280 , a representative embodiment of which is shown in FIG.  5 . The centering scope  280 , when attached to the controlled end  206  of the manipulator  200 , desirably includes a cross-hair reticle or other suitable “optical finder” that can be trained on the reticle or cross-hair target of the centering gauge  148  and thus be used as an optical locating and centering device. The centering scope  280  includes a self-indexing adapter block  282  fitted with a male dovetail rail segment  284  configured to slide into and be held in the female dovetail socket  226  of the electrode manipulator  200 . The centering scope  280  includes an eyepiece lens  286 , an optical tube  288 , and an objective lens  290 . The centering scope  280  can have any convenient magnification, such as  20   x  magnification, sufficient to obtain, for example, accurate alignment of the optical axis O Z  of the centering scope with the axis PAX Z  of the stereotaxic holder  10  (such alignment is shown in FIG.  4 ). After performing the alignment, the centering scope  280  can be detached from the controlled end  206  and a new implement attached to the controlled end with the longitudinal axis O Z  of the implement automatically being aligned accurately with the axis PAX Z . Further detail on use of the centering scope  280  is provided later. 
     A second example implement is a drilling unit  300  for use in drilling a hole in a subject animal&#39;s skull or for performing analogous tasks in preparation for implanting a probe at the desired locus in the subject body, or for any of various other surgical purposes. A representative embodiment of a drilling unit  300  is shown in FIG. 6, and includes a self-indexing adapter block  302 , a male dovetail rail segment  304 , motor  306 , housing  308 , and chuck  310  adapted to hold, e.g., a drill bit  312 . Normally, the drilling unit  300 , when mounted to the controlled end  206  of the electrode manipulator  200 , presents the drill bit  312  coaxially with the axis O Z  of the implement (e.g., the centering scope  280 ) previously attached to the controlled end. 
     A third example implement is a syringe holder  330  adapted to hold a surgical or microinjection syringe. A representative embodiment of a syringe holder  330  is shown in FIG. 7, and includes a self-indexing adapter block  332 , a male dovetail rail segment  334 , a syringe enclosure  336  configured and dimensioned to hold a particular type of syringe  338 , an adjustable “zeroing” scale  340 , and a needle guide tube  342 . Normally, the syringe holder  330  presents a hollow needle  344  or probe to be inserted, along the axis O Z , into the desired locus in the subject animal. 
     A fourth example implement is a dial test indicator unit  360  used for determining and calibrating the alignment of the axis O Z , such as whether the axis O Z  is oriented exactly perpendicularly to the surface of the plate  132  (or of the plate  182 ) and whether all three axes of the electrode manipulator  200  are exactly perpendicular to each other and/or exactly aligned with the corresponding Cartesian axes of the stereotaxic holder  10 . Such determinations and calibrations are similar to analogous determinations and calibrations, respectively, (termed “sweeping in” or “indicating”) performed with three-axis machine tools. A representative embodiment of a dial test indicator unit  360  is shown in FIG. 8, and includes a self-indexing adapter block  362 , a male dovetail rail segment  364 , a shaft  366  having an axis O Z  alignable with or relative to the axis PAX Z  of the stereotaxic holder  10 , an arm  368  that is oriented angularly relative to the axis O Z  in an adjustable manner, and a dial indicator  370  (e.g., LAST WORD™ indicator, model 711-MF, manufactured by Starrett, Athol, Mass.) including a contact point  372 . The shaft  366  is rotatable relative to the adapter block  362 , and can be manipulated to move (raise and lower) the position of the arm  368  (with dial indicator  370 ) along the axis O Z . A collar  374  can be cinched onto the shaft  366  to hold the shaft  366  at a particular position along the axis O Z  relative to the adapter block  362 . A threaded shaft  376  (to which a knurled nut  374  is threaded) cinches the arm  368  at a desired angular orientation relative to the shaft  366 . 
     As an example protocol with which the dial indicator can be used, the dial indicator is mounted to the controlled end  206  with the shaft  366  oriented vertically downward toward the surface of the plate  182 . The contact point  372  is placed in contact with the surface of the plate  182 . The user observes the numerical reading on the dial indicator  370  while rotating the shaft  366  about the axis O Z . If the displayed numerical value changes with angle of rotation of the shaft  366 , axial adjustment can be performed by turning the jack screws  238 ,  240  (FIG. 4) as required until the dial indicator  370  reads the same value with any angle of rotation. 
     A fifth example implement is any of various cannula-insertion devices. A first embodiment  400  of a cannula-insertion device is shown in FIG.  9 . The FIG. 9 embodiment  400  is relatively simple and comprises a self-indexing adapter block  402 , a male dovetail rail segment  404 , a shaft  406  having an axis O Z  alignable with or relative to the axis PAX Z  of the stereotaxic holder  10 , and a cannula-holding arm  408  configured to hold a cannula  410  (or analogous tool) such that a longitudinal axis thereof is aligned with the axis O Z . A cinching screw  412  affixes the cannula  410  to the terminus of the arm  408 . The FIG. 9 embodiment  400  can be used to hold and implant one cannula tube (or analogous tool) to a desired on-plane locus. 
     A second embodiment  420  of a cannula-insertion device is shown in FIG.  10 . The FIG. 10 embodiment  420  is especially suitable for holding and implanting one or two cannulae (or analogous tools) to respective on-plane loci. The FIG. 10 device comprises a first cannula holder  422  for holding a first cannula  423  (or other tool shaped similarly to a cannula) and a second cannula holder  424  for holding a second cannula  425  (or other tool shaped similarly to a cannula). Mounted in their respective holders  422 ,  424 , each cannula  423 ,  425  can be placed at different respective X-axis and Y-axis coordinates. More specifically, the first cannula  423  held in the first cannula holder  422  is aligned longitudinally with the axis O Z  (and thus directly alignable with or relative to the PAX Z  axis of the stereotaxic holder  10 ). The second cannula  425  held in the second cannula holder  424  can be positioned relative to the first cannula  423  (while remaining parallel to the first cannula  423 ) by manipulating one or both of a first shift mechanism  426  and a second shift mechanism  428  described in more detail below. 
     The cannula-insertion device  420  comprises a self-indexing adapter block  430  including a male dovetail rail segment  431 , a shaft  432  inserted into the adapter block  430  and having an axis O Z , the first and second cannula holders  422 ,  424 , respectively, and the first and second shift mechanisms  426 ,  428 , respectively. The first shift mechanism  426  comprises a first member  434  attached to the shaft  432 , first and second parallel guide bars  435 ,  436 , respectively, affixed to the first member  434 , and a second member  438  adapted to slide along the guide bars  435 ,  436 . One or more extension springs (not shown) desirably are situated between the first and second members  434 ,  438  to urge the members to move together. A force counter to the spring force is applied by a first micrometer head  440  which, when turned, controllably adjusts the spacing (along the indicated X-axis) between the first and second members  434 ,  438 , and thus the spacing (along the indicated X-axis) between the first and second cannulae  423 ,  425 . The second shift mechanism  428  comprises a member  442  to which first and second guide bars  444 ,  445 , respectively, are affixed. The guide bars  444 ,  445  slide relative to the member  438 . One or more extension springs (not shown) desirably are situated between the members  438 ,  442  to urge the members to move together. A force counter to the spring force is applied by a second micrometer head  446  which, when turned, controllably adjusts the spacing (along the indicated Y-axis) between the members  438 ,  442  and thus the spacing (along the indicated Y-axis) between the first and second cannulae  423 ,  425 . The member  434  terminates with a clamp  447  adapted to grip the first cannula  423  whenever the screw  448  is tightened. Similarly, the member  424  terminates with a clamp  449  adapted to grip the second cannula  425  whenever the screw  450  is tightened. On the opposite side of the adapter block  430  is a collar  452  attached to the adapter block  430  and coaxial with the axis O Z . The shaft  432  terminates with a knurled knob  454  that, when turned, rotates the entire cannula-insertion device relative to the adapter block  430  about the axis O Z  (i.e., about the indicated Z-axis). The angular orientation of the cannula-insertion device about the axis O Z  can be locked by tightening a cinching screw (not shown) threaded through the collar  452  to engage the shaft  432 . 
     A third embodiment  470  of a cannula-insertion device is shown in FIG. 11, which has especial utility for independently holding and implanting one or two cannulae to respective off-plane loci. The FIG. 11 device  470  comprises the following components that are similar to corresponding components (described above) in the FIG. 10 embodiment  420 : self-indexing adapter block  472 , male dovetail rail segment  473 , and shaft  474 . The FIG. 11 device  470  comprises a first cannula holder  480  for holding a first cannula  481  (or other tool shaped similarly to a cannula) and a second cannula holder  482  for holding a second cannula  483  (or other tool shaped similarly to a cannula). The first and second cannulae  481 ,  483 , respectively, are held in first and second cannula adapters  484 ,  485 , respectively, mounted to respective first and second cannula holders  480 ,  482 , respectively. When so mounted, the terminus of the first cannula  481  and the terminus of the second cannula  483  can be placed at different respective X-axis and Y-axis coordinates by manipulating one or both of a first shift mechanism  486  and a second shift mechanism  487 . Further detail regarding mounting the cannulae  481 ,  483  in the respective cannula adapters  484 ,  485 , and mounting the cannula adapters  484 ,  485  in the respective cannula holders  480 ,  482  is provided later below. 
     The first shift mechanism  486 , similar to the first shift mechanism  426  of the FIG. 10 embodiment  420 , comprises a first member  488  attached to the shaft  474 , first and second parallel guide bars  489 ,  490 , respectively, affixed to the first member  488 , and a second member  491  adapted to slide along the guide bars  489 ,  490 . One or more extension springs (not shown) desirably are situated between the first and second members  488 ,  491  to urge the members to move together. A force counter to the spring force is applied by a first micrometer head  492  that, when turned, controllably adjusts the spacing (along the indicated X-axis) between the first and second members  488 ,  491 , and thus the spacing (along the indicated X-axis) between the terminus of the first cannula  481  and the terminus of the second cannula  483 . The second shift mechanism  487  comprises a member  493  to which first and second guide bars  494 ,  495 , respectively, are affixed. The guide bars  494 ,  495  slide relative to the member  491 . One or more extension springs (not shown) desirably are situated between the members  491 ,  493  to urge the members to move together. A force counter to the spring force is applied by a second micrometer head  496  that, when turned, controllably adjusts the spacing (along the indicated Y-axis) between the members  491 ,  493  and thus the spacing (along the indicated Y-axis) between the terminus of the first cannula  481  and the terminus of the second cannula  483 . 
     To the member  488  is affixed a first arc plate  497 , and to the member  493  is affixed a second arc plate  498 . The first cannula holder  480  is attached to the first arc plate  497 , and the second cannula holder  482  is attached to the second cannula holder  482 . The first cannula holder  480  comprises a slide mechanism comprising a plate  499 , a block  500  adapted to slide relative to the plate  499  as controlled by a lead screw  501  (manually turned using a knurled knob  502 ), and a tool clip  503  configured to grip the first cannula adapter  484  (or other suitably shaped tool). Thus, turning the knurled knob  502  controllably shifts the cannula  481  (or other tool) along a first cannula axis C 1 . The plate  499  can be adjustably moved along the arc defined by the first arc plate  497  so as to change the angle of the first cannula axis C 1  relative to the axis O z  (or to a line parallel to O z ). Similarly, the second cannula holder  482  comprises a slide mechanism comprising a plate  504 , a block  505  adapted to slide relative to the plate  504  as controlled by a lead screw  506  (manually turned using a knurled knob  507 ), and a tool clip  508  configured to grip the second cannula adapter  485 . Thus, turning the knurled knob  507  controllably shifts the cannula  483  along a second cannula axis C 2 . The plate  504  can be moved adjustably along the arc defined by the second arc plate  498  so as to change the angle of the second cannula axis C 2  relative to the axis O z . Furthermore, the respective angles of the cannula axes C 1 , C 2  relative to O z  (or to respective lines parallel to O z ) need not be the same and can be adjusted independently. 
     Whenever the slide mechanism of the first cannula holder  480  is shifted fully downward, the first cannula  481  or other tool (held in the first cannula adapter  484  mounted to the first cannula holder  480 ) desirably is situated such that the terminus of the first cannula  481  (or other tool) is situated exactly on the axis O z  (and thus directly alignable with the PAX z  axis of the stereotaxic holder  10 ). To such end, the tool clip  503  and block  500 , functioning in combination with the first cannula adapter  484 , desirably are “self-indexing,” as follows. FIG. 12 shows a first cannula adapter  484  (detached from the first cannula holder  480 ) placed in a “gauge block”  900 . The first cannula adapter  484  includes a shoulder portion  902  having a facing surface  904 . The gauge block  900  is used to establish a standard length (L) from the facing surface  904  to the terminus  906  of the cannula  481 . The first cannula adapter  484  (with cannula  481  or other tool attached but with the screw  908  loosened) is placed in the gauge block  900  such that the facing surface  904  contacts a first surface  910  of the cannula adapter. Meanwhile, the terminus  906  of the cannula  481  is placed in contact with a hardened region  912  of a second surface  914 . Afterward, the screw  908  is tightened to fasten the cannula  481  to the cannula adapter  484 . The cannula adapter with attached cannula can be removed from the gauge block  900  and mounted to the first cannula holder  480  (FIG. 11) such that the facing surface  904  contacts the upward-facing surfaces of the tool clip  503  and the block  500 . Whenever the cannula adapter  484  is mounted in such a manner to the first cannula holder  480  (with the slide mechanism of the first cannula holder  480  fully shifted downward), the terminus  906  of the first cannula  481  (or other tool) is situated exactly on the axis O z  (as shown in FIG.  11 ), and thus directly alignable with or relative to the PAX z  axis of the stereotaxic holder  10 . Any other tool mounted to the first cannula adapter  484  in the manner described above will also have its terminus contact the axis O z . 
     Desirably, the second cannula adapter  485  is “self-indexing” with respect to the second cannula holder  482  in the same manner as discussed above. It also will be appreciated that a cannula  483  or other tool can be mounted to the second cannula adapter  485 , and the second cannula adapter mounted to the second cannula holder  482 , in the same manner as described above regarding the first cannula. In any event, the terminus of the second cannula  483  (held in the second cannula adapter  485  mounted to the second cannula holder  482 ) can be positioned relative to the terminus of the first cannula  481  by manipulating one or both of the first shift mechanism  486  and the second shift mechanism  487 . 
     Whenever the first cannula  481  has been mounted in a self-indexing manner as described above, so as to place the terminus of the first cannula on the axis O Z , the plate  499  can be moved adjustably along the arc defined by the first arc plate  497  so as to change the angle of the first cannula axis C 1  relative to the axis O Z  (or to a line parallel to O Z ) without changing the location, in three-dimensional space, of the terminus of the first cannula  481 . Similarly, whenever the second cannula  483  has been mounted in a self-indexing manner as described above, the plate  504  can be moved adjustably along the arc defined by the second arc plate  498  so as to change the angle of the second cannula axis C 2  relative to the axis O Z  (or to a line parallel to the axis O Z ) without changing the location, in three-dimensional space, of the terminus of the second cannula  483 . 
     As with the FIG. 10 embodiment, the FIG. 11 embodiment can be provided with a knurled knob and collar (corresponding to the knob  454  and collar  452  of the FIG. 10 embodiment). If such a knob is provided, turning the knob would cause rotation of the entire cannula-insertion device  470  relative to the adapter block  472  about the axis O Z  (i.e., about the indicated Z-axis). 
     The FIG. 11 embodiment  470  can be used to hold any of various tools other than cannulae. For use, the cannula-insertion device  470  is mounted to the controlled end  206  of the manipulator  200  and positioned at a desired location relative to the subject body. Before mounting a cannula (mounted to its respective cannula adapter) to the device  470 , a miniature drilling device can be mounted to the respective cannula holder  480 ,  482  for drilling a hole through which the subject cannula is to be inserted. After drilling the respective hole, the drilling device is detached from the cannula holder and replaced with the respective cannula (in its respective cannula adapter). 
     The drilling device (or any other tool mounted to a cannula holder  480 ,  482 ) desirably is self-indexing in the same manner as the respective cannula adapter  484 ,  485 . Any of various self-indexing tools can thus be attached wherein the terminus of the tool is always situated (whenever the corresponding cannula holder is shifted to its full-down position) at exactly the same position in three-dimensional space. This advantageously avoids having to perform repositioning each time a new tool is mounted to the cannula-insertion device  470 . 
     Based on the previous discussion, it will be appreciated that any of the slide and shift mechanisms of the embodiments of the cannula-insertion devices described above can be substituted with any of various alternative mechanisms. Furthermore, the knurled knobs need not be actuated manually. Rather, it will be immediately apparent that actuation of one or more slide or shift mechanisms can be automated by using motors or the like instead of the knurled knobs. 
     A sixth example implement is a stereotaxic alignment indicator, of which a representative embodiment  520  is depicted in FIG.  13 . When mounted to the controlled end  206  of the manipulator  200 , the stereotaxic alignment indicator can provide dimensional feedback to the user required to obtain a desired adjustment/alignment of coronal tilt and dorsal tilt of a body mounted to the stereotaxic holder  10 . The embodiment  520  of FIG. 13 comprises a self-indexing adapter block  521 , a male dovetail rail segment  522 , a shaft  523  inserted into the adapter block  521  and having an axis O Z , a knurled knob  524  attached to the shaft  523 , and a collar  525 . Turning the knurled knob  524  causes rotation of the entire stereotaxic alignment indicator  520  relative to the adapter block  521  about the axis O Z . The angular orientation of the stereotaxic alignment indicator  520  about the axis O Z  can be locked by tightening a cinching screw (not shown) threaded through the collar  525  to engage the shaft  523 . 
     The shaft  523  is affixed to an angled block  527 . On a distal edge of the angled block  527  is mounted a bilateral slide mechanism  528 . The bilateral slide mechanism  528  comprises a center block  529  and opposing flanking blocks  530 ,  531 . Parallel guide bars  532 ,  533  are affixed to and extend bilaterally from the center block  529  through the flanking blocks  530 ,  531 . A threaded shaft  534  (with oppositely pitched threads on each half) extends bilaterally from the center block and is threaded into the flanking blocks  530 ,  531 . Thus, turning a knurled knob  535  attached to an end of the threaded shaft  534  causes the flanking blocks  530 ,  531  to move synchronously toward or away from the center block  529 . To each flanking block  530 ,  531  is mounted a respective vertical slide mechanism  536 ,  537 . Each vertical slide mechanism comprises a pair of parallel guide bars  538   a ,  538   b  and  539   a ,  539   b , respectively. The guide bars slide vertically relative to the respective flanking block  530 ,  531 , and terminate with a respective pin bar  540 ,  541  affixed to the respective guide bars  538   a ,  538   b  and  539   a ,  539   b , respectively. Attached to each pin bar  540 ,  541  is a respective contact pin  542 ,  543 . Mounted to the angled block  527  are first and second dial indicators  544 ,  545  for the first and second slide mechanisms  536 ,  537 , respectively. Each of the dial indicators  544 ,  545  has a stem that extends through the respective flanking block  530 ,  531  and a respective tip  546 ,  547  that contacts the respective pin bar  540 ,  541 . 
     During use, the terminus of each contact pin  542 ,  543  is placed in contact with the surface of a subject body. Normally, the force of gravity (together with the relatively weak spring bias of the respective tip  546 ,  547 ) provides sufficient bias to the pin bars  540 ,  541  for the respective contact pins  542 ,  543  to remain in contact with a test surface. The vertical position of one contact pin relative to the other pin can be ascertained by reading the dial indicators  544 ,  545 . I.e., a change in the vertical position of a contact pin  542 ,  543 , causes a corresponding change in the deflection of the respective tip  546 ,  547 . As is generally known with a dial indicator of the type shown, whenever the tip of the dial indicator is displaced a corresponding change is caused in the dimensional value indicated by the dial indicator. In the FIG. 12 embodiment, the tip  546 ,  547  of each dial indicator  544 ,  545  contacts the upper surface of the respective pin bar  540 ,  541 . Thus, a change in the vertical position of a contact pin  542 ,  543  is translated to a change in the vertical position of the respective pin bar  540 ,  541 , thereby changing the dimensional value displayed by the respective dial indicator  544 ,  545 . For ease in calibration, the dial of each dial indicator  544 ,  545  is adjustable to a desired null value as desired or required. Dial indicators (e.g., LAST WORD™ indicators, model 711-MR, manufactured by Starrett, Athol, Mass.) having an accuracy sufficient for use with rodent skulls desirably have an accuracy of +/−10 μm. 
     The lateral gap between the contact pins  542 ,  543  can be adjusted as required by turning the knurled knob  535 . The obtained lateral gap is equilateral relative to the axis O Z  (i.e., regardless of the spacing between the pins  542 ,  543 , each pin is an equal distance from the axis O Z ). The dimension of the actual gap can be ascertained by consulting a vernier scale  548 . 
     During use, the stereotaxic alignment indicator  520  is mounted to the manipulator  200  as described above. Generally, the alignment indicator  520  is first oriented such that a line connecting the termini of the contact pins  542 ,  543  is parallel with the axis PAX X  of the stereotaxic holder  10 , as shown in FIG.  14 ( a ). By turning the knob  216  on the manipulator  200 , the alignment indicator  520  is lowered down onto the surface of the subject body structure (e.g., skull, not shown) being held by the stereotaxic holder  10  until the contact pins  542 ,  543  contact the surface of the body structure. The gap between the contact pins  542 ,  543  can be adjusted appropriately, by turning the knob  535 , to the desired value to contact the desired bilateral loci on the body structure. For example, if the body structure is a rodent skull, then the contact pins  542 ,  543  can be adjusted to contact bilateral loci flanking the sagittal suture or to correspond with the actual distance between bregma and lambda. To achieve a level aspect of a line extending between the points of contact of the indicator probes with the body structure, the knob  98  of the stereotaxic holder  10  is adjusted, as described above, until both dial indicators  544 ,  545  read exactly the same value or both indicate a “null” value. Alternatively, adjustment is made to achieve a desired tilt (other than level) of the subject body structure, as indicated on the dial indicators  544 ,  545 . 
     To achieve alignment in the other of the X- and Y-axes, the stereotaxic alignment indicator  520  is raised off the body structure (by turning the knob  216 ), rotated 90 degrees by turning the knob  524 , and lowered again onto the body structure (by turning the knob  216 ). Thus, a line connecting the termini of the contact pins  542 ,  543  is now parallel with the axis PAX Y  of the stereotaxic holder  10 , as shown in FIG.  14 ( b ). For example, after aligning the sagittal suture of a rodent skull parallel with the PAX Y  axis, the alignment indicator  520  is lowered onto the skull until one of the contact pins  542 ,  543  contacts bregma and the other contact pin contacts lambda. The knob  120  on the stereotaxic holder  10  is turned to adjust the dorsal tilt of the skull until both dial indicators  544 ,  545  display the same value or a null value, or a desired differential value. As a result of this adjustment, a line extending between bregma and lambda along the sagittal suture is level or at the desired angular orientation to within, e.g., +/−10 μm. 
     With respect to an alignment indicator, any of various alternative embodiments to FIG. 13 embodiment are possible. For example, and not intending to be limiting, the dial indicators  544 ,  545  can be replaced with any of various digital scales, such as those discussed elsewhere herein. Further alternatively, the mechanical vertical slide mechanisms  536 ,  537  (with associated dial indicators  544 ,  545 ) can be replaced with a “touch signal probe” as known in the art or with one or more laser position detectors. 
     As discussed above, an appropriate snout adapter  160  or other implement for holding a subject body can be attached to the base  14  of the first U-frame  12 . (See generally FIG.  1 ( b ) showing a representative embodiment of a snout adapter  160  attached to the mounting rods  152 ,  154 .) An appropriate snout adapter is particularly useful when the subject body is a head or skull. In view of the many differences in skull size and shape among various possible subject animals, the appropriate snout adapter will have a correspondingly different configuration. Snout adapters are used usually in conjunction with other head-holding implements that usually include ear bars  20 ,  22  as shown in FIGS.  1 ( a )- 1 ( b ). A combination of a snout adapter and ear bars provides a three-point contact system for the subject skull, and three-point contact systems are especially effective for holding the skulls of smaller rodents such as mice, rats, squirrels, and the like. Heads of larger animals such as cats, dogs, and primates frequently need at least one other contact point for adequate stability. For such heads, “eye bars” (that engage the infra-orbital ridge) are used frequently in addition to tooth bars and ear bars. Of course, if the body being held is not a head or skull, the implements used to grasp the body have other respective configurations each of which desirably conforming to a respective anatomical structure so as to provide a stable point of contact. 
     The embodiment of the snout adapter  160  shown in FIGS.  1 ( a ) and  1 ( b ), which is especially suitable for holding a rodent skull, is detailed in FIG.  15 . The FIG. 15 snout adapter  160  is especially suitable for use in conjunction with ear bars, such as the ear bars  20 ,  22  shown in FIGS.  1 ( a )- 1 ( b ), appropriately sized for the subject skull. The snout adapter  160  comprises a mounting block  562  and a snout-engagement portion  563 . 
     The mounting block  562  defines apertures  564  through which the mounting bars  152 ,  154  (FIG.  1 ( b )) extend. A lock screw  565  can be tightened for locking the mounting block  562  at a desired location on the mounting bars  152 ,  154 . Thus, whenever the snout adapter  160  is mounted to the first U-frame  12 , the snout adapter  160  is movable relative to the first U-frame  12  substantially along the Y-axis (i.e., to provide a desired anterior-posterior adjustability). 
     The snout-engagement end  563  comprises a snout-clamp/gas-mask  566  and a palate bar  567 . The palate bar  567  defines a through aperture  568  sized to allow the subject&#39;s incisors to extend therethrough. The palate bar also defines lateral recesses  569  configured and situated to contact the subject&#39;s molars, allowing the subject&#39;s palate to rest on a mid-line ridge  570 . Whenever the palate bar  567  is thus engaged with the palate of the subject, the snout-clamp/gas-mask  566  can be moved posteriorly relative to the mounting block  562  along the indicated Y-axis to fit over the subject&#39;s nose (i.e., the subject&#39;s nose is inserted into a cavity  571  defined by the snout-clamp/gas-mask  566 ), thereby “clamping” the subject&#39;s snout. After a desired fit is obtained, a locking screw  572  is tightened. The snout-clamp/gas-mask  566  is also tiltable about the indicated Y-axis, relative to the mounting block  562 . The particular tilt can be retained by tightening the locking screw  572 . 
     The snout-clamp/gas-mask  566  also comprises a gas inlet  573  and a gas outlet  574  to allow administration of a gas anesthetic to the subject while the subject&#39;s head is engaged in the snout adapter  160 . More specifically, the gas inlet  573  is connectable to a supply of anesthetic gas. The gas outlet  574  is connectable, for example, to a waste-gas reservoir maintained under a slight subatmospheric pressure. 
     As noted above, FIG. 15 shows a representative embodiment of a snout adapter. Any of various other snout adapters as currently known in the art readily can be adapted for mounting to the stereotaxic holder  10 . Example conventional snout adapters are available from, for example, Kopf Instruments, Tujunga, Calif. (e.g., model  926  “mouse adapter,” model  920  “rat adapter,” model  924  “rotational rat adapter,” and model  906  “rat anesthesia mask”). 
     Whereas apparatus according to the invention are especially adapted for holding a body (i.e., animal body or portion thereof) for performing a surgical or diagnostic intervention, for example, it will be appreciated that the subject “body” is not limited to animate bodies. In fact, any of various inanimate “bodies” or other workpieces can be held and aligned in a stereotaxic manner using apparatus according to the invention. 
     A representative protocol for performing a stereotaxic alignment is set forth below as performed using a rodent skull as a representative body structure. In this protocol, it is assumed that the stereotaxic holder  10  and the manipulator  200  are attached to the base  180  as described above. Also, this example protocol is described in the context of the specific embodiments shown in the figures described above. It will be understood that details of the protocol may change with changes, for example, in the specific embodiment that is used and in the particular subject. 
     (1) If required, the orthogonality of the X-, Y-, and Z-axes of the manipulator  200  are checked. This can be performed, e.g., by mounting the dial test indicator  360  to the controlled end  206  of the manipulator  200 , performing “sweeping-in” or “indicating” as described earlier above, and adjusting the jack screws  238 ,  240 ,  242  on the universal joint  230  of the manipulator as required. 
     (2) The dial test indicator  360  is detached from the controlled end  206  and replaced with the centering scope  280 . The gauge post  146  is placed on the pad  150 . The centering scope  280  is positioned, using the manipulator  200 , so that the reticle in the scope is aligned exactly with (and focused on) the centering gauge  148  on the gauge post  146 . This action establishes coincidence of the axis O Z  of the centering scope  280  (and thus of the controlled end  206 ) with the axis PAX Z . Also, by focusing the scope  280  on the centering gauge  148 , the point on the PAX Z  axis where the axes PAX X  and PAX Y  cross each other is established. If the manipulator  200  is equipped with digital scales  220 ,  222 ,  224 , each scale desirably is nulled at this time. In any event, with respect to both the stereotaxic holder  10  and the manipulator  200 , a “0,0,0” point is identified in three-dimensional space (i.e., the point where the axes PAX X , PAX Y , PAX Z  orthogonally cross each other). The 0,0,0 point is the reference point from which various loci in or on the subject body are located accurately in three-dimensional space. After the 0,0,0 point is located, the gauge post  146  is removed. 
     (3) The skull is mounted to the stereotaxic holder  10  using a proper combination of holding implements such as ear bars and snout adapter. If desired, the controlled end  206  of the manipulator can be moved out of the way. The advantage of previously having nulled the scales  220 ,  222 ,  224  is immediately apparent because the controlled end  206  can be returned with high accuracy to its previous position simply by adjusting the knobs  214 ,  216 ,  218  until all three scales  220 ,  222 ,  224  return to their respective null values. In any event, after mounting the skull to the stereotaxic holder  10 , the centering scope  280  is returned to the 0,0,0 position and the axis O Z  is made coincident with the PAX Z  axis. 
     (4) While observing through the centering scope  280 , the skull is shifted (using the shift mechanisms  28 ,  30 ,  32  as required), to place the desired target feature at the 0,0,0 point (i.e. at the cross-reticle of the centering scope in all three dimensions). For a rodent skull, the target feature is often bregma. However, as noted earlier above, any of various other target features on or in the body can be used, including artificially implanted features. 
     (5) While still observing through the centering scope  280 , a desired anterior-posterior reference line (e.g., a natural linear feature such as the sagittal suture of the skull) is aligned with the Y-direction reticle line in the centering scope. Alternatively, for example, regarding bregma as a first reference point, the centering scope can be shifted (by manipulating the Y-direction shift mechanism  208 ) to the lambda locus on the skull, and the “swing” of the stereotaxic holder  10  can be adjusted (using the knob  140 ) as required to align an imaginary anterior-posterior reference line connecting bregma and lambda on the subject skull exactly with the PAX Y  axis. 
     (6) Using the Y-direction shift mechanism  208 , the controlled end  206  is shifted to a position at which the axis O Z  intersects the anterior-posterior line of the skull at midlength, such as midlength between bregma and lambda. 
     (7) The centering scope  280  is removed, and the stereotaxic alignment indicator  520  is attached to the controlled end  206  of the manipulator  200 . Normally, coronal tilt of the subject skull is determined first. This can be done by lowering the contact pins  542 ,  543  onto respective points on the skull that are located bilaterally relative to the anterior-posterior reference line. The knob  98  on the stereotaxic holder  10  can be adjusted as required to obtain either a level line connecting the two bilateral points or to obtain a line at the desired coronal tilt angle. 
     (8) The stereotaxic alignment indicator  520  is retracted from the skull (using the Z-axis shift mechanism  210  of the manipulator) sufficiently to allow a 90-degree rotation (using the knob  524 ) of the alignment indicator  520 . Thus, the alignment indicator is positioned for ascertaining the dorsal tilt of the subject skull. The gap between the contact pins  542 ,  543  is set appropriately (using the knob  535 ), for example to equal the bregma-lambda distance. The alignment indicator is then lowered until the contact pins  542 ,  543  contact the skull on the anterior-posterior reference line. The dorsal tilt of the skull is adjusted (by manipulating the knob  34  on the stereotaxic holder  10 ) until the desired readings (level or otherwise) are obtained on the dial indicators  544 ,  545 . For example, some rodent brain atlases locate features of the brain relative to bregma and lambda being level; other atlases locate features relative to a 2.25-mm offset of bregma to lambda. Either adjustment can be made readily in this step. 
     Upon completing steps (1)-(8), the subject skull is now positioned in a true stereotaxic plane according to the pertinent reference (brain atlas or other appropriate reference), with a pre-determined degree of confidence based on the accuracy of the indicators (dials, scales, etc.) provided on the apparatus according to the invention. The alignment indicator can be retracted from the skull and replaced with any of various implements attached to the controlled end so as to continue with the surgery or other research intervention involving the subject skull. For example, any of various electrodes, cannulae, probes, etc. can be implanted to desired respective loci within the skull (e.g., within the brain) at a high level of confidence that the desired loci will, in fact, be “hit.” 
     Whereas the invention has been described in connection with representative embodiments, it will be apparent that the invention is not limited to those embodiments. On the contrary, the invention is intended to encompass all modifications, alternatives, and equivalents as may be included within the spirit and scope of the invention, as defined by the appended claims.