The human hand has twenty-nine bones and twenty-nine major joints. It contains over one hundred ligaments, connected to thirty-four muscles in the palm alone, each one responsible for the minute negotiations that allow us to tie our shoes, thread a needle, lift a glass, or juggle. Thirty arteries pump blood alongside them. Forty-eight nerves—running from the spine under the clavicle, through the armpit, and down the length of the arm—conduct the electrical signals that let us turn a wrist, crook a finger, or make a fist. A quarter of the brain’s motor cortex is devoted to our hands.

On a rainy Friday in February, a woman briskly worked a glove over my right hand and fastened a plastic ring onto my index finger. I sat on a stool in a windowless room, facing a rig studded with motion capture cameras angled inquisitively toward me. A large screen commanded my attention: when the light turns green , it read, use your wrist to control the cursor on the screen.…