Canavero has a very clear picture of this surgery in action, having outlined it in two TED talks, in a keynote address last summer at an American conference of neurosurgeons, and in a new book, Il Cervello Immortale (“The Immortal Brain”). He describes it to me in detail: the operating theater of the near future, where two bodies will be clamped tight in special frames. One will be the anesthetized patient; the other, a brain-dead donor. Using either “a specially fashioned diamond microtomic snare-blade” or “a nanoknife made of a thin layer of silicon nitride” (he isn’t sure yet), the bodies will be severed at the neck between the C5 and C6 vertebrae. Then the frames that are clamping the two bodies will begin to separate, their upper parts rotating and taking the two heads with them. The patient’s head will be deposited atop the donor’s body.

Next, a marathon of surgery, somewhere between 36 and 72 hours long and requiring a crew of 150 medics. About 80 of them, Canavero thinks, will need to be surgeons. “At first, it will be expensive” — around $17 million, he guesses, admitting that private sponsors are still needed. “Later, as the technique gets perfected, the costs will be slashed.” In the operating room, those 80 surgeons will relay in and out as expertise dictates. The head-body arteries will be joined first, so that blood recirculates around the brain. As for the other connections required (windpipe, gullet, spine, everything that links a human’s head to the rest), Canavero says he will stand aside until it comes to the spinal cord. Functional neurosurgery, or that relating to movement, is his field.

If his patient is to regain movement after the operation, some of the millions of nerves that exist inside the two spinal cords will need to connect.