TECHNICAL FIELD The present invention relates to an incisal guiding table having a variable arcuate guiding surface. BACKGROUND ART Prosthetic articulators are instruments for making dental prostheses, and must be set according to the specific anatomical conformations of the patient to reproduce the patient's teeth as faithfully as possible. The specific anatomical conformations of the patient are obtained using ordinary measuring instruments, such as a caliper, axiograph, or pantograph. Prosthetic articulators comprise a top portion, to which the upper jaw model is fixed, and a bottom portion, to which the lower jaw model is fixed; the top and bottom portions are connected by two articulations simulating the patient's condyloid articulations; and various inserts are fitted selectively to the articulation, for this purpose, to simulate the patient's condyloid articulation cavities. Prosthetic articulators also comprise an incisal guiding table and an incisal rod, which cooperate with each other and, together with the condyloid articulation, serve to define the conformation of the teeth. The incisal guiding table comprises a slide having a guiding surface along which the end of the incisal rod slides in use, and the inclination of which defines the inclination of the tooth being defined. It is commonly acknowledged, and widely confirmed in literature, that the anatomical conformation of the condyloid articulations has absolutely no effect on the anatomical conformation of the front teeth. Which is why incisal guiding tables with a substantially flat guiding surface have always been used up to now. Since guiding tables of this sort obviously prevent formation of a correct palatal portion using the articulator, it is common practice that, once the temporary prosthesis is implanted in the patient's mouth, the precise conformation of the various palatal portions of the teeth is defined in a number of consultations, and, once the right conformation is supposedly achieved, this is reproduced in the final prosthesis. The above method is a possible source of discomfort to the patient who, for a given time, is fitted with a temporary prosthesis which does not match his own teeth. Moreover, the final conformation of the prosthesis is left to the manual skill of the operator and to the not always reliable sensitivity of the patient. A need is therefore felt for an incisal guiding table designed to provide a straightforward, low-cost solution to the problems posed by the known art. DISCLOSURE QF INVENTION According to the present invention, there is provided an incisal guiding table comprising a guiding surface, and setting means for setting the inclination of said guiding surface; said incisal guiding table being characterized in that said guiding surface is of arcuate shape. In a preferred embodiment of the present invention, the incisal guiding table comprises adjusting means for adjusting said arcuate shape. BRIEF DESCRIPTION OF THE DRAWINGS A non-limiting embodiment of the invention will be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a view in perspective of a preferred embodiment of the incisal guiding table according to the present invention; Figure 2 shows a side view of the Figure 1 incisal guiding table. BEST MODE FOR CARRYING OUT THE INVENTION Number 1 in Figures 1 and 2 indicates as a whole an incisal guiding table in accordance with a preferred embodiment of the invention. Incisal guiding table 1 comprises a fastening portion 2, in turn comprising a circular plate 3, and a parallelepiped-shaped structure 4 fixed to plate 3. Circular plate 3 and parallelepiped-shaped structure 4 both have a circular opening 5. Incisal guiding table 1 comprises two lock screws 6 (only one shown) , each cooperating with a respective nut screw (not shown) formed in parallelepiped-shaped structure 4. Incisal guiding table 1 comprises a movable portion 7 made of plastic material and in turn comprising a supporting portion 8, and a slide 9 extending from supporting portion 8. Supporting portion 8 is connected to parallelepiped-shaped structure 4 by means of lock screws 6. More specifically, supporting portion 8 comprises two lateral appendixes 8a, in which are formed two curved guides 10, each housing a respective lock screw 6. Guides 10 and the action of screws 6 on supporting portion 8 provide for altering and securing the angle of slide 9. That is, once the angle of slide 9 is selected, lock screws 6 are tightened to grip lateral appendixes 8a between parallelepiped-shaped structure 4 and the heads 6a of screws 6. Slide 9 comprises a guiding surface 11; a nut screw ' 12 on the opposite side to surface 11; and a screw 13 cooperating with nut screw 12, and the tip 13a of which rests against supporting portion 8. By virtue of the elasticity of movable portion 7 and the presence of nut screw 12 and screw 13, the arcuate shape of coiiding surface 11 can thus be adjusted continuously as required. An incisal guiding table as described above has the enormous advantage of producing, by means of the articulator on which it is mounted, a much more accurate prosthesis than those obtainable using known incisal guiding tables . In fact, once the patient's individual anatomical conformation has been determined using the measuring instruments, the incisal guiding table according to the present invention provides for setting not only the specific condyloid articulations and the specific angle of the slide, but also a specific arcuate conformation to obtain a prosthesis identical with the actual conformation of the patient's teeth. This therefore eliminates the drawbacks typically associated with temporary prostheses, in which the palatal portion of the teeth cannot be defined using known incisal guiding tables. Moreover, as opposed to trial and error application based on patient sensation, the final conformation of the palatal portion of the tooth is an accurate reproduction based on measurements taken beforehand. Though providing means by which to continuously adjust the arcuate conformation of the guiding surface makes for an infinitely versatile incisal guiding table, the present invention also provides for a number of inserts, each representing a guiding surface with a specific arcuate conformation, and from which, in use, the insert most closely resembling the patient's particular conformation is selected. Finally, the incisal guiding table according to the present invention comprises known rotation means, not described, for rotating at least the slide about an axis perpendicular to the fastening portion, to orient the slide accurately for all types of front teeth.