The Xing were a humanoid species who were well known for their surgeons. Xing surgeons were trained by being awoken in the middle of the night and having to attach one of their big toe. Xing were also able to shoot barbs from their chin. These barbs could knock out a human.
Aldridge was a Xing surgeon who had a practice on Gallifrey. He left around the time the First Doctor did because he found the Time Lords too pretentious. He settled in London during the late 18th century afterward. (PROSE: A Big Hand for the Doctor)