Sensory adaptation, the same phenomenon that causes you to stop hearing a continually ticking clock, also prevents you from seeing the branching network of blood vessels inside your eye. Except when you use the simple trick demonstrated in this video.
The video was posted by thegnome54, a YouTuber studying cognitive neuroscience at Brown University. All you need to do it at home is an index card punched with a pinhole (our narrator shows you how to use just your finger too) and a uniform source of light, like a blank web page in your browser. Because your brain is used to seeing the shadows cast by the blood vessels inside your eye day after day, you stop seeing them. And presumably that happens shortly after birth.
But this simple trick, which involves looking through the small hole while it quickly moves up and down, changes those shadows just enough so that your brain recognises them again. Allowing you to actually visualise the vessels criss-crossing your eye like the branches of a tree. Actually seeing them for the first time is a little freaky, but I was even more impressed at how quickly your brain starts ignoring them again when the hole stops moving. [YouTube via Geeks are Sexy]