Hi there.
It's been considered very important throughout known history and is found throughout esotericism.
This link which talks about spinal fire can help a little and lead to other helpful resources.
http://www.hermetics.org/stavish/Secret_Fire.html
David Wilcock talks about it amongst other things in this vid. I think he has read Manly Hall on this.
http://video.google.co.uk/videosearc...%20gland&emb=0
You suppress it by making sure 99% of people aren't aware of what
it's potential is.
You can't just switch it on by thinking about it or meditating either.
It's a discipline as you'll discover.
This site is also great for anyone interested in how peoples of the past
viewed the human potential. It holds not only texts and manuscripts but
scholarly works on the subjects and it's all free.
http://www.sacred-texts.com/index.htm
There is a voluntary option to buy a CDRom with all the info on.
Hope this helps.
Cheers, Chris