Trephine apparatus has an outer casing with means defining a substantially cylindrical passage, or aperture, opening at a substantially circular lower end element, and an inner, substantially cylindrical tube having a bore opening at a substantially circular lower end element, the lower end element of the inner tube being upwardly spaced, or recessed, from the lower end element of the outer casing within the passage. A substantially cylindrical trephine blade is mounted within the bore of the inner tube by means that is so constructed as to enable raising and lowering of the blade relative to the end elements of the outer casing and the inner tube. At least a lower end element of the outer casing, or a base portion including it, is made of a non-opaque material, enabling edge-on viewing of the basal element of the inner tube and the blade, such as by use of a microscope, without parallax between the blade and the basal element of the inner tube. The zero position of the blade can be set easily and highly accurately, in a manner that is not possible with existing vacuum trephine assemblies.
