In 2002, a Pittsburgh neuropathologist named Bennet Omalu, a native of Nigeria, examined the body of 50-year-old former Pittsburgh Steelers center Mike Webster. At the end of his life, Webster had suffered a steep mental decline, becoming violent, depressed and forgetful and pushed to increasingly desperate lengths to battle chronic pain. In Webster’s brain, Dr. Omalu, who holds multiple advanced degrees and certifications from top American medical schools, discovered what would mark a turning point in the evolution of thinking about the effects of head injuries in professional football. The following excerpt is from...