From chapter one: The Sea monster Your memory is a monster; you forget--it doesn''t. It simply files things away. It keeps things for you, or hides things from you--and summons them to your recall with a will of its own. You think you have a memory; but it has you!--John Irving, A Prayer for Owen Meany At the bottom of the ocean, tail curled around seagrass, the male seahorse sways back and forth in the current. He may be tiny and mysterious, but no ocean creature compares to him. The only male in the animal kingdom to become pregnant, he stands on guard, carrying his eggs in his pouch until they hatch and the fry swim away into the open sea. But let''s back up: this isn''t a book about seahorses. To find our real subject, we must rise out of the depths and journey back 450 years.

The year is 1564. We''re in Bologna, Italy, a city full of elegant brick buildings and shady, vine-covered walkways. Here, at the world''s first proper university, Dr. Julius Caesar Arantius bends over a beautiful object. Well, beautiful might be an exaggeration, if you''re not already deeply, passionately involved in its study. It''s a human brain. Rather gray and unassuming, and on loan from a nearby mortuary. Students surround the doctor, clustered on benches throughout the theater, following his work intently, as though he and the organ in front of him are the two leads in a drama.

Arantius leans over the brain and slices through its outer layers, studying each fraction of an inch with extreme interest, hoping to understand what it does. His disregard for religious authority is clear in the gusto with which he approaches his dissection because, at the time, the scientific study of human corpses is strictly forbidden. The doctor cuts further into the object, examining what''s inside. And then, deep within the brain, buried in the temporal lobe, he finds something very interesting. Something small, curled up into itself. It looks, he thinks, a bit like a silkworm. The upper classes of the Italian Renaissance loved silk, a luxurious and exotic fabric that arrived in Venice via the Silk Road from China; by extension, they loved silkworms too. Intrigued, Arantius looks closer, making some careful cuts, and pries the little worm loose, liberating it from the rest of the brain.

This is the moment at which modern memory research was born, the precise moment that memory, as a concept, moved from the mythological world into the physical one. However, back then, on that particular day in sixteenth-century Bologna, life goes on in the markets as usual; people carry wine and truffles and pasta below the city''s famous pergolas and ancient red brick towers, oblivious to the hugely important discovery in their midst. Arantius turns over what he has dug out of the brain and places it on the table before him, considering what it might be. That''s it! Rather than a silkworm, perhaps it is a tiny seahorse? Yes, indeed. With its head nodding forward and its tail curling up, it does look like a seahorse, the tiny distinctive fish living in shallow ocean waters between the tropics and England. And so he names it: hippocampus, meaning "horse sea monster" in Latin. It also shares its name with a mythological creature--half horse, half fish--said to wreak havoc in the waters around ancient Greece. By the light of a tallow candle perched on an autopsy table, Julius Caesar Arantius couldn''t tell what this little part of the brain actually did.

All he could do was give it a name. Hundreds of years passed before we fully understood the significance of what this Italian doctor held in his hands, and you might guess that it has something to do with memory. After all, memory is the subject of this book. The world beneath the sea and the one in our brain are profoundly different, of course, but there are many similarities between the seahorse and the hippocampus. Just as the male seahorse carries his eggs in his pouch until it''s safe for the fry to be on their own, the seahorse of the brain also carries something: our memories. It watches over them and nurtures them until they are strong enough to make it on their own. The hippocampus is the womb that carries our memories. No one knew how crucial the hippocampus was to memory until 1953, but there was endless speculation about where memories were stored in the brain.

One popular early belief was that our thoughts flowed through the liquid inside our skulls, but that theory was long gone by 1953. By then, the prevailing thought was that memories were created and stored throughout the brain. But then something happened to sink this theory once and for all, an incident that was tragic for one man, fortunate for the rest of us. An unsuccessful experimental surgery was the key to understanding Julius Caesar Arantius''s earlier discovery.