Abstract: An instrument similar in size and use to a cystotome, combining into a single instrument two knives which can be independently used so that the cutting surfaces of the knives can cut, at right angles to the surface to be incised, in either of two directions, one direction being at an angle with respect to the other direction. The knives are located at the ends of coaxially disposed elongate members formed from hypodermic needles. A first of the knives is guarded by the other knife until relative rotation of the needles brings the second knife into use. The inner one of the needles is connected to a hypodermic needle hub which in turn is connected to a hypodermic syringe, thus allowing fluids to be injected into or withdrawn from the surgical site. Apparatus suitable for causing vibration of the knives is included.