Abstract: A guide for a medical instrument includes a cranial tap having a threaded end adapted to be secured into the skull of a patient, a spherical member having a passageway extending diametrically through it for receiving an instrument employed in a surgical procedure, and retaining structure attachable to the tap to immobilize the spherical member in a desired orientation within the tap, characterized in that the retaining structure includes first and second threaded members secured to the tap in different locations for engaging the spherical member in different locations to thereby retain the spherical member against movement within the tap.