A retractor having interchangeable or replaceable blades facilitates the expansion of a surgical site below an incision in the skin, and establishes a work-through surgical space. The retractor includes a frame connected to two collar arms, a pair of blades connected to the collar arms, a sheath for encompassing the blades and providing a circumscribed working space, and a device for rotating the collar arms, thereby displacing the blades and any tissue surrounding them.