Amazon offered me this curious book for four bucks, so I bought it without further inspection.
When it arrived, I beheld its total of 9 pages of text, and admit to having felt robbed. It's but one chapter from a whole book.
That aside, the text is densely put to the point, and my initial regret is remedied. My prior knowledge about ancient Egyptian surgery, Greek philosophy, general biology in vertebrate and evolution and so on, though all of a mediocre level, helped a lot in interpreting the given references and explorations.
In short, almost all vertebrate have a third eye. It's still somewhat functional in lizards, but in most others it has degenerated. In humans, some tests show that the gland/eye responds to stimuli by vibrating. The learned mystic can experience and provoke this vibration through discipline and exercise. Though I doubt ancient surgeons could perform such a test on a living human as we could today, it was no difficulty for them to find the gland in a brain and deduce its meaning through observation and analogy - they cut open reptiles too, you see, and drew references from what they learned.
Since the gland is situated between the sides of the brain, it's assumed it functions as a central conduit for all senses, for the left and the right, the ratio and emotio - reason and intuition, the factual and the ephemeral.
Philosophies of all times tried to discipline the individual into harmonizing both, his/her reason and intuition, knowledge and faith, to acquire some sort of personal ascendance. Once found, the gland was held as the seat of the soul, of the mind, the place where all this harmonizing and transcending occurs in the body. I am somewhat familiar with the idea behind the so-called philosopher's stone, and was not surprised to find its organic representative here.
A final detail: The gland is a pine shaped thing hanging on a cord, connected to the spine. As mentioned before it reacts to stimuli by vibration, but also by swelling and the cord can stiffen. It amuses me greatly to see that so many symbols of old (staffs with cone top, erect feathers, etc.), today thought to have phallic origin, less than a hundred years ago where still recognized as connected to this part of the brain all humans share, independent of gender or race.
As for so many things about the brain, our technology is still too inferior to measure and represent their exact function, nature and meaning. Incapable of illuminating or eradicating any of the herein given hypotheses, we are stuck once more at dealing with the unknown, the unsure, the obscure, or, to use a very outdated word of the very same thing, the occult.