Abstract: A first dilator (10) and second dilator (20) each comprise an elongate hollow member (11; 21) with a “closed” end (12; 22) and an “open” end (13; 23). The outer surface of the “closed” end (12; 22) is of uniform arcuate section and of generally hemispherical configuration. The “open” end (13) of the member (11) is provided with internal threading (16), while complementary external threading (24) is provided on the “open” end (23) of the member (21). In use, the first dilator (10) and second dilator (20) are joined together by engaging the threading (16; 24). A flange (25) provided on the second dilator (20) provides a continuously smooth outer surface.