But Dr. Annese has something Dr. Yakovlev did not: advanced computer technology that tracks and digitally reproduces each slice. An entire brain produces some 2,500 slices, and the amount of information in each one, once microscopic detail is added, will fill about a terabyte of computer storage. Computers at U.C.S.D. are now fitting all those pieces together for Mr. Molaison’s brain, to create what Dr. Annese calls a “Google Earthlike search engine,” the first entirely reconstructed, whole-brain atlas available to anyone who wants to log on.

Image LAYERS “I feel like the world is watching over my shoulder,” said Jacopo Annese, who dissected the brain of Henry Molaison, known as H. M. Credit... Diego Mariscal/The Brain Observatory, University of California, San Diego

“We’re going to get the kind of resolution, all the way down to the level of single cells, that we have not had widely available before,” said Donna Simmons, a visiting scholar at the Brain Architecture Center at the University of Southern California. The thin whole-brain slicing “will allow much better opportunities to study the connection between cells, the circuits themselves, which we have so much more to learn about.”

Experts estimate that there are about 50 brain banks in the world, many with organs from medical patients with neurological or psychiatric problems, and some with a stock donated by people without disorders. “Ideally, anyone with the technology could do the same with their own specimens,” Dr. Corkin said.

The technical challenges, however, are not trivial. To prepare a brain for dissection, Dr. Annese first freezes it in a formaldehyde and sucrose solution, to about minus 40 degrees Celsius. The freezing in the case of H. M. was done over four hours, a few degrees at a time: the brain, like most things, becomes more brittle when frozen. It can crack.

Mr. Molaison lost his ability to form new memories after an operation that removed a slug-size chunk of tissue from deep in each hemisphere of his brain, making it more delicate than most.

“A crack would have been a disaster,” Dr. Annese said. It did not happen.

With the help of David Malmberg, a mechanical engineer at U.C.S.D. who had designed equipment for use in the Antarctic, the laboratory fashioned a metal collar to keep the suspended brain at just the right temperature. A few degrees too cold and the blade would chatter instead of cutting cleanly; too warm, and the blade wants to dip into the tissue. Mr. Malmberg held the temperature steady by pumping ethanol through the collars continually, at minus 40 degrees. He suspended the hoses using surfboard leashes picked up days before the dissection.