At nine o’clock on July 28th last year, Wendy Nissley carried her two-year-old daughter, Lacy, into O.R. 12 at Johns Hopkins Hospital to have half of her brain removed. Lacy suffers from a rare malformation of the brain, known as hemimegalencephaly, in which one hemisphere grows larger than the other. The condition causes seizures, and Lacy was having so many—up to forty in a day—that, at an age when other toddlers were trying out sentences, she could produce only a few language-like sounds. As long as Lacy’s malformed right hemisphere was attached to the rest of her brain, it would prevent her left hemisphere from functioning normally. So Lacy’s parents had brought her to Johns Hopkins for a hemispherectomy, which is probably the most radical procedure in neurosurgery.

Wendy laid her daughter on the operating table. Because Lacy was so small, it took the anesthesiologist almost ninety minutes to insert her intravenous lines. George Jallo, the attending neurosurgeon, spent a long time arranging her head on gel padding and then drew “Cut here” markings on her shaved scalp. The rest of Lacy’s head, including her face, was covered with a sterile drape. Jallo made one long cut across the top of her head from the front to the back, and another at right angles to the first, which started midway along it and stopped just in front of her right ear. He folded back the scalp and made small holes in her skull with a power drill, outlining a rough semicircle. Then he used the drill to connect the dots and removed a portion of the skull. He cut another T in the dura, a thin, leathery membrane covering the brain. Gently, he peeled back two large flaps.

By half past one, Jallo and a resident had already removed the right frontal lobe. David Lieberman, the pediatric neurologist who had examined Lacy when she first came to Johns Hopkins, looked on, shaking his head in wonderment. “It’s so open,” he said, turning to me. “Normally, with brain surgery, you make a hole about this big”—he curled his thumb and index finger into a circle.

After removing the frontal lobe, Jallo embarked on the parietal lobe. In case complications put a sudden stop to the surgery, it was important to take out the seizure hot spots first, gradually working through the hemisphere in descending order of priority: after the parietal lobe would come a small section of the occipital lobe, then the temporal lobe, then the rest of the occipital. Finally, Jallo would cut the corpus callosum, the bundle of fibres that connect the two hemispheres of the brain. The surgeons slowly worked around each side of the parietal lobe, making tiny pinches in the brain with electric cauterizing forceps. There was a slight smell of burning in the bright, noisy operating room. As the cut became deeper and wider, the tissue on either side browned and blackened, and the lobe started to move back and forth. At the bottom of the parietal wedge, the clean white of nerve fibres was visible; as the lobe was severed, they came apart like string cheese. A surgical technician bent toward Jallo with a small plastic bowl in his hands. Jallo picked the lobe out of the skull—it was the size of an infant’s fist—and dropped it into the container.

As she led me out of the O.R., Eileen Vining, the attending neurologist, said, “Did you see how rigid it was? Normal brain sags in your hands.” Vining talked quickly, moving from one complicated idea to the next, punctuating each with “O.K.?” and an expectant nod. She had been in and out of the operating room all morning, and now she was off to find the Nissleys and tell them how Lacy was doing.

Four hours later, Vining took me back into the O.R. Lacy’s right hemisphere was gone, and her cranium looked like a wide, uneven bowl. I could see the deep cavity where the frontal and parietal lobes had been, and the white-pink color inside the base of the skull. In the middle of the remaining brain was a shallow mound where Jallo had left a layer of nerve fibres to protect the ventricle, a fluid-filled pocket that cushions the brain and the spinal cord. The white matter there was now gray-black. Jallo and his resident lightly touched their forceps to it, and the cauterizers fizzed, sealing the brain to prevent microhemorrhages. Hemorrhaging is a constant concern in brain surgery, and at one point in the operation Jallo decided to leave in a small piece of the right occipital lobe which threatened to bleed dangerously. Jallo glanced at Vining and Lieberman, and the doctors stretched forward to look at the severed corpus callosum. Over and over, the surgical technician poured in saline, and Jallo and his resident drew it out again with a loud suction pump.

When he had finished removing brain tissue, Jallo tipped in small packets of Surgicel, a feathery white substance that helps blood to clot. It melted onto the surface of the brain. “That was good. There was not a lot of bleeding,” Vining said. “You never know what you are going to get until you open it up. Sometimes you just go in there and you hold your breath and pray.”

In the final hour, Jallo sutured Lacy’s dura, which had shrunk slightly from dehydration, and filled the right side of her head with saline. The technician then brought the missing section of the skull back to Jallo, carrying it in a wide right angle over surgical carts rather than risk moving it over the floor. Jallo reattached it, using four tiny dissolvable plates made of a sugarlike substance. He then closed the scalp incision with a staple gun, leaving seventy-eight aluminum staples in Lacy’s skin. The hemispherectomy had taken nine hours. The resident bandaged Lacy’s head, gently turned her onto her right side, and stuck a piece of tape on her head that said “This side up.”

I first met the Nissleys three months before Lacy’s surgery, at their home in Palmyra, Pennsylvania. As I drove up, I could see Lacy’s father, Mike, dangling a leg over the edge of the front porch. He looked like a surfer, with a goatee beard and gold-lens Oakleys pushed back on his head. Wendy sat on a chair near the front door, and Lacy’s four-year-old sister, Lily, flitted around the adults. Lacy, blond like her mother, lay across Wendy’s lap. “She had a seizure just before you got here, so she’s sleeping now,” Wendy said. They invited me inside.

Lacy had her first seizure on October 13, 2003, when she was four and a half months old. Wendy had been doing laundry, and when she turned around she saw that Lacy’s lips were blue and that she was shaking. She called 911, but by the time the ambulance arrived the seizures had ended. In the following month, the family went to the hospital many times, but, on each occasion, by the time they got there the spell had ended and there didn’t seem to be anything wrong. The doctors took an MRI but saw nothing unexpected, and they suggested that Lacy might have acid reflux or sleep apnea, or could even be doing it on purpose, out of anger. “But she wasn’t angry,” Mike said. “She was sitting there happy, and all of a sudden it hit her like a train.” One doctor put her on phenobarbital, increasing the dose each time there was another seizure. “It made her lethargic, like she was drunk,” Wendy said. That November, Lacy had a terrible attack. “Her arm went up in the air,” Mike said, throwing his left arm straight up, stiff. “Her lips went blue. She was having them in a row.” By the time the ambulance came, the seizures had subsided, but the attendants advised the Nissleys to get in their car and take Lacy to Johns Hopkins anyway. The drive took two hours, and, just as Mike and Wendy walked over to the triage nurse, Lacy began another cluster of seizures. The nurse said, “We’ve got to get you a room.”