You Have An Eye Inside Your Head

Quick: How many eyes do you have? You probably said two. But human beings all have what has been called a vestigial (unused) eye in their brains. This eye has a structure similar to our external eyes, with retinal tissues, and it is sensitive to light.

This eye, known as the pineal gland, has been much-discussed in philosophy and medicine. There are a few explanations of the role played by the eye in your brain - from it's function as a 'third eye' that can perceive things we cannot see with our external eyes to its role in producing melatonin and more.