Disclosed is a medical valve comprising a pair of plates holding in tension a membrane folded over to form a chamber with an elongated, slit-like opening along adjoining edges. The plates include interlocking members which interlock the plates together. An inlet tube in communication with the chamber extends outwardly from the plates. The preferred configuration of the chamber is trapezoidal. Also disclosed is a surgical instrument that is a needle-like member having an elongated slot in the wall of this member. A method of using this instrument to implant a tubular element in the body of a patient is also disclosed.