In that pantheon of illuminatingly broken men and women, Henry stands apart. It is difficult to exaggerate the impact he has had on our understanding of ourselves. Before Brenda Milner collaborated on that first paper about Henry, the prevailing theory of memory held that its functions could not be localized to a single cortical area, that learning was distributed across the brain as a whole. By that theory — built upon the experimental lesioning of the brains of rats — a person’s memory would be affected only in proportion to the amount of brain tissue removed, regardless of which brain structures the tissue was removed from. Milner’s first paper about Henry, along with her previous work, upended this view. She demonstrated, with elegance and rigor, that Henry’s amnesia was profound — possibly the most catastrophic she had ever seen — and declared that it must have been a result only of the relatively small and specific bilateral lesions to his hippocampus and other medial temporal structures left by the operation. This was an astonishing revelation. It was not the last.

Five years after Milner’s first paper about Henry, she published a second that was almost as revelatory. That paper documented Henry’s gradual improvement over a three-day period on a difficult hand-eye coordination task. His improvement came despite his inability to ever remember his previous attempts at the task, indicating that there are at least two different memory systems in the brain — one responsible for our conscious, episodic memories, the second responsible for task-or-skill related “procedural” memories — and that these two systems seem to rely on entirely distinct parts of the brain. This was another fundamental step forward in our understanding of how memory works. Together, Milner’s two Henry-­related revelations can be viewed as the cornerstones of modern memory science.

After Corkin took over the research, the revelations kept coming, though now they were of a smaller scale. Corkin and her colleagues added fine detail to the portrait of Henry’s damaged condition, filling in gaps, but the weight of their work could seem slight when compared with Milner’s monumental achievements. Corkin and her colleagues learned that if you placed a pain-­inflicting device called a dolorimeter to Henry’s chest, he wouldn’t complain even as his skin began to turn red and burn. She learned that if you presented him with two dinners in a row, he would eat them both, because by the time he began the second he would have forgotten the first. She learned that he was apparently asexual and that there was no evidence of his ever even masturbating. She learned the ins and outs of his temperament, the frequency of his tantrums, the patterns of his infrequent complaints. She noted the odd exceptions to his amnesia, like the fact that Henry, after years of watching the sitcom “All in the Family,” eventually came to know that Archie Bunker’s son-in-law was called Meathead. Corkin cataloged his verbal tics, his malapropisms, his stock phrases.

“I’m having an argument with myself,” Henry would say, over and over and over again.

As the experiments piled up and the data accumulated, Henry became a boon not just to science but also to Corkin’s career. She started her own lab at M.I.T., and although she and her colleagues conducted research in a number of areas, the papers that generated the most attention were always the ones about Henry. When they first met, Corkin was a young graduate student in her 20s. She grew older. She became a renowned professor of neuroscience at one of the world’s greatest universities. Henry grew older, too, though he wasn’t exactly aware of it. In Henry’s later years, people were always asking him how old he thought he was, and he would make a series of guesses. Was he in his 30s, his 40s, his 50s? He had only the vaguest sense of the passage of time. Then someone might pass him a mirror and watch him gaze into his own elderly eyes.

“I’m not a boy,” he would say, finally.

Henry died on a winter afternoon in 2008. The next morning, Corkin peered through a window into an autopsy room at Massachusetts General Hospital, watching as two men cut off the top of Henry’s skull. For 46 years, Corkin had been having her one-­sided meetings with Henry, endlessly introducing herself to an old friend. Now she was having one last encounter that only she would remember. The men carefully pulled out Henry’s brain, and Corkin gazed at it through the glass, marveling at this object she had spent her career considering at one step removed.

Later, reflecting on that moment, Corkin could think of only one word to describe her feelings. She was, she wrote, “ecstatic.”