THE DEEPEST CUT.

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."