But if sensory feedback can extend a surgeon’s body across a room, why stop there? A new version of the machine adds Ethernet, freeing the doctor to inhabit a mechanical body anywhere with a good cable or wireless connection. By digitizing surgical commands, we’ve already created transitional moments in which maneuvers have been described but not executed. Why not extend this transition, playing out the surgery in virtual reality and then editing out any errors? That’s the next step: surgery with a word processor, so to speak, instead of a typewriter.

Unfortunately, the military doesn’t have these luxuries. Soldiers get wounded in faraway places without broadband or doctors, and they need help fast. That’s why Darpa wants mobile machines that can do surgery without human guidance. Such robots are in the works, according to Bel­fiore. Their initial repertory will be limited, but that’s O.K. They just have to keep the wounded alive for the hour it takes to reach a hospital. And with every life they save, they’ll begin to earn our trust.

So maybe you’ll let a robot fix your body. But would you let one join your body? In fact, the coupling is well under way. As troops come home from Iraq and Afghanistan with limbs blown off, they get computerized arms that read the body’s electrical signals. They’re cyborgs.

The next step is mutual adaptation. Amputees have always had to learn how to operate their new limbs. Now the limbs are returning the favor. Their software studies each user’s electrical signals, gradually becoming more accurate at interpreting commands. And though the user’s brain remains in charge, his body has become negotiable. Amputees are getting surgeries to make their motor signals more readable by myoelectric arms. The human is being reconfigured for the machine.

The eventual payoff isn’t just parity with unreconstructed humans. It’s superiority. Some mechanical arms now exceed the reach of human arms. Last year, a disabled sprinter was forbidden to run in Olympic-level track meets on his carbon-fiber legs because they were deemed too fast. And computerized limbs can be networked. Belfiore recalls a recent conversation with an Iraq war amputee about whether his new hand could manipulate a mouse. “Why do I need a mouse?” he asked. “Why can’t I plug my arm right into a USB port?”

For that matter, who needs a USB port? Limb designers have devised injectable sensors that can transmit motor commands to artificial arms through wireless signals. Once you can operate an arm wirelessly, you don’t need it attached to your body. You can control it from anywhere.

But your arm can also be hacked. And that raises an unsettling question: If humans marry machines, who will control the marriage? In its 2007 car contest, Darpa took elaborate measures to stop robots from going rogue. Each vehicle was outfitted with multiple shutdown devices and trailed by a human driver with a kill switch. The penalty for the slightest disobedience was immediate disqualification. But at least one team, according to Belfiore, liked to run simulations with its car’s “software aggression level cranked up into what they jokingly called Rambo mode.”

Imagine your arm in Rambo mode. Something like that has already been reported: Michael Weisskopf, a journalist who lost his right hand in Iraq, was making a turn in rush-hour traffic sometime later when, as Belfiore describes it, Weiss­kopf’s new hand “clenched the wheel of his car in a death grip and refused to let go.” It was just a misunderstanding. But electronic limbs are being programmed to make more and more decisions. After all, it isn’t just your body anymore. It’s theirs, too.