Bicycling to the hospital, Henry Marsh writes, he is oppressed by “a feeling of doom.” Photograph by Tom Pilston / Panos

For the schoolteacher, the changes had come slowly. First, his walking had grown unsteady; then his hearing had worsened. He had become stooped, and had begun walking with a cane, even though he was only in his late fifties. Now he sat with his wife and son in the consulting room of Henry Marsh, a London neurosurgeon, looking at a scan of his brain, which showed a tumor growing near the base of his skull. The question was whether it could, or should, be removed. Marsh, who had been practicing neurosurgery for only a few years, was unsure. The tumor was massive—he was startled by its size—and it was situated in the brain stem, a vital area. Left to itself, it would destroy the schoolteacher’s hearing, rob him of his ability to walk, and, eventually, kill him. But, Marsh explained, surgery could leave him paralyzed, or worse. The family faced a difficult choice, between the certainty of a slow, predictable decline and the possibility of an immediate cure—or catastrophe.

They decided to seek a second opinion from an older, eminent neurosurgeon. A few days later, the surgeon phoned Marsh. “It’s a young man’s operation,” he said. “I’ve told them you should do it.” Flattered, Marsh agreed to go ahead. The surgery began at nine in the morning and continued late into the night. Brain surgery is slow and dangerous, and removing a tumor can be like defusing a bomb. Often, surgeons look through a microscope and use long-handled, fine-tipped instruments to pull the tumor away from the brain before removing it with a sucker. A quarter of the body’s blood courses through the veins and arteries of the brain; if one of them is torn, bleeding and stroke can result. It’s also possible to remove important parts of the brain by accident, because brain tissue and tumor tissue look pretty much the same. Unlike the rest of the body, the brain and the spinal cord rarely heal. If a neurosurgeon makes a mistake, the damage is often permanent.

By midnight, Marsh and his team had removed almost all of the tumor. The atmosphere in the operating theatre was relaxed and celebratory; the surgical team paused for cigarette breaks and listened to Abba and Bach. “I should have stopped at that point, and left the last piece of tumor behind,” Marsh writes in his memoir, “Do No Harm” (Thomas Dunne). Instead, he ventured further—he wanted to be able to say that he had taken it all out. “As I started to remove the last part of the tumor,” Marsh writes, “I tore a small perforating branch off the basilar artery, a vessel the width of a thick pin. A narrow jet of bright red arterial blood started to pump upwards.” The basilar artery carries blood to the brain stem, which regulates the rest of the brain. Marsh quickly stopped the bleeding, but the oxygen deprivation was enough to irreparably damage the man’s brain stem, and he never regained consciousness.

Marsh, who is now sixty-five, is one of Britain’s foremost neurosurgeons. He is a senior consultant at St. George’s Hospital, in London, and he helped to pioneer a kind of surgery in which patients are kept awake, under local anesthesia, so that they can converse with their surgeons while they operate, allowing them to avoid damaging what neurosurgeons call “eloquent,” or useful, parts of the brain. Marsh has been the subject of two documentary films. Still, he writes, “As I approach the end of my career I feel an increasing obligation to bear witness to past mistakes I have made.” A few years ago, he prepared a lecture called “All My Worst Mistakes.” For months, he lay awake in the mornings, remembering the patients he had failed. “The more I thought about the past,” he recalls in his book, “the more mistakes rose to the surface, like poisonous methane stirred up from a stagnant pond.”

There’s a tradition of physicians writing about their errors. “When the Air Hits Your Brain,” a neurosurgical memoir by Frank Vertosick, Jr., begins with a scene in which a resident, while drilling a hole in a man’s skull, accidentally goes too far, plunging the drill bit into the brain. “Oh, shit!” he exclaims. (An older doctor reassures him: “It’s just the lateral hemisphere.”) Physician writers usually view such errors with a generous spirit. They point out that medicine is built on mistakes, because doctors, like the rest of us, learn by screwing up.

Marsh isn’t interested in the usefulness of error. He is the Knausgaard of neurosurgery: he writes about his errors because he wants to confess them, and because he’s interested in his inner life and how it’s been changed, over time, by the making of mistakes. As an epigraph to “Do No Harm,” he quotes the French doctor René Leriche: “Every surgeon carries within himself a small cemetery, where from time to time he goes to pray.” Marsh knows there’s something unprofessional about this inwardness—a surgeon’s emotions are supposed to be beside the point compared with his patients’ suffering—but he is drawn to “reckless honesty.” (When he delivered “All My Worst Mistakes” to an audience of neurosurgical colleagues, he writes, “it was met by a stunned silence and no questions were asked.”) “Do No Harm” is an act of atonement, an anatomy of error, and an attempt to answer, from the inside, a startling question: How can someone spend decades cutting into people’s brains and emerge whole?

Marsh became a neurosurgeon almost by accident. Midway through his undergraduate years, at Oxford, he fell in unrequited love and, inspired by the Jack Nicholson movie “Five Easy Pieces,” fled to Newcastle, in the rugged northeast of England, to nurse his broken heart. There, he wrote bad poetry, worked as a hospital porter, and saw his first surgery. “I found its controlled and altruistic violence deeply appealing,” he writes. After he finished his degree, in 1973, he entered the Royal Free Hospital School of Medicine. Students weren’t allowed into the neurosurgical theatres, but one day Marsh caught a glimpse through a round porthole in a closed door—“a naked woman, anaesthetized, her head completely shaven, sitting bolt upright on a special operating table.” The image stayed in his mind, and struck him as “a scene from a horror film.”

Marsh married, and qualified as a doctor. Not long afterward, his three-month-old son, William, developed a tumor in the center of his brain and successfully underwent surgery to remove it. Marsh feels now that he didn’t fully appreciate the risks: he writes that, much later, “I watched a child bleed to death in the very same operating theatre where my son had been treated, as my boss—the very surgeon who had saved my son’s life—now failed with a similar tumor.” Soon after his son’s surgery, while working in intensive care, Marsh observed an aneurysm operation. The surgeon had to make his way deep into the brain, exposing the small, deadly balloon of arterial blood so that, without rupturing it, he could seal it off using a miniature metal clip. It “was more like a blood sport than a calm and dispassionate technical exercise,” Marsh writes. It also “involved the brain, the mysterious substrate of all thought and feeling. . . . The operation was elegant, delicate, dangerous, and full of profound meaning. What could be finer, I thought, than to be a neurosurgeon?” Neurosurgery—strange, brutal, and miraculous—had seduced him, and he started the training as soon as he could.