The nail protrudes at an oblique angle from a length of wood wedged into the canyon floor. The tip meets my leg about two inches above the ankle, and the force of the jump drives it straight through my calf. There’s a searing pain. I look down in time to see the flesh closing swampily around the nail and realize with horror that a piece of wood about a foot and a half long is now attached to my leg. Instinctively, I take it in both hands and, with as much strength as I can muster, pull it out again. The tissue surrenders the nail with a sucking noise and the blood follows – at first in a great gush, then in long powerful spurts like the initial yield of an unexpected oil strike. I press one thumb on it, then the other, but the blood flows relentlessly down my leg, seeping into the white cotton of my sock like tomato juice into kitchen paper. Shocked, upping the pressure of my thumbs, I try to remember if there’s an important artery there. One of those bleeding-to-death-in-three-minute ones. As an experiment, I let go. Sure enough the blood pulses steadily higher. Tourniquet! I think wildly, tourniquet! – And squeeze down as hard as possible.