The invention relates to a surgical appliance for forming an opening through the skin of a patient and for distending the opening so formed. The appliance takes the form of a trocar which may comprise two separate components or two component portions formed by openings in a tube extending partially along the tube from the distal end. In either case, the components or component portions are preloaded towards a closed condition, but can be expanded into an open condition, for example by forcing a tubular expander into the trocar from the proximal end. The preloading of the trocar may be by a resilient sleeve surrounding the components or by a "set" introduced into the material forming the components. A cutting edge may be formed across substantially the full width of the distal end of the trocar and it is preferred that in the open condition, there is a no significant increase in the cross-section of the trocar towards the distal end.