In July, 2015, Stephen Casper, a healthcare historian, received a shocking e-mail from a workforce of legal professionals. They have been representing a team of retired hockey gamers who were suing the Nationwide Hockey League their match argued that the N.H.L. had failed to alert them about how regime head punches and jolts in hockey could set them at danger for degenerative mind problems. The legal professionals, unusually, desired to hire a historian. A sort of dementia called chronic traumatic encephalopathy, or C.T.E., experienced not too long ago been posthumously identified in dozens of previous professional football and hockey players diagnosable only by a mind autopsy, it was thought to…