When a human being runs, we have a tiny, little neck that emerges from the center of the base of our skull, and it’s very short in the middle. We’re basically like pogo sticks. We’ve lost, by becoming bipeds, all those mechanisms available to quadrupeds to keep their heads still. It turns out that we’ve evolved other special mechanisms to keep our heads still. One of them, the semicircular canals (the vestibular system in our heads) are especially enlarged, and give us enormous sensitivity to pitching forces, to pitching motions. The semicircular canals, the vestibular system are organs of balance that essentially function as an accelerometer. As your head pitches forward, as it does every time you hit the ground when you run, your head wants to pitch forward. As it pitches forward, the enlarged semicircular canals - these are the anterior and posterior ones, for anybody who actually cares - are especially large. That gives them greater gain in their sensitivity to angular accelerations. Which then, through a three-neuron circuit to our brain activates, without any conscious effort, the eye muscles that actually then stabilize the gaze. So even when your eyes are closed and you move your head, your eyes, the semicircular canals, through that three-neuron system operates those muscles, keeping your gaze stabilized. It’s that fundamental a system.