Which structure in the brain, removed from patient H.M., ended his ability to form new memories?

The hippocampus, taken from both sides in 1953 in an attempt to control severe epilepsy. Henry Molaison was 27. The seizures improved; he never formed another lasting conscious memory, and for the remaining 55 years of his life he greeted the researchers who had worked with him for decades as strangers.

What he could still do is what made him the most studied patient in the history of neuroscience. His childhood memories were intact, so the hippocampus is not where memories are stored but how they are laid down. His working memory was fine over seconds. And he learned motor skills perfectly well - practised at tracing a shape in a mirror, he improved day after day while insisting each time that he had never done it before - which proved that skill memory and fact memory are separate systems in separate places.

He consented to the study of his brain after death, and in 2009 it was sliced into 2,401 sections in a procedure streamed live. The consent question hangs over the whole case: a man who could not remember agreeing to anything was studied for half a century on the authority of a guardian, and the ethics of that are now taught alongside the findings.