Muscae Volitantes
Those little threads that float in your field of vision when you stare at a light-colored area are the remnants of the (pre-natal) Hyaloid artery. Have you ever noticed how they appear to move somewhat independently of the rest of your vision, coasting to a near-stop some seconds after your eyes have already found their focus?

This makes me think that they'd be a good start to setting up some kind of optical interface for an implanted object tracker...

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