According to Holmes, who was Dr. Mortimer?

After examining the walking stick, Holmes concludes that Mortimer received the stick as a gift from a hospital, rather than from a hunting club. He also deduces that Mortimer was a student at the hospital, not a physician, and that he must therefore be young, not old. Furthermore, he believes that Mortimer has withdrawn from a town hospital to begin his own practice in the country. He adds that Mortimer must be absent-minded, amiable, unambitious and a dog owner.