Japanese traditional culture ~5~ torii

I write about torii on this column.
Do you remember I attach the photo of torii when I wen to Akagi
natural park?
Here is the photo.


This object is called torii
tori = bird
i = ? it means "exist" (It is hard to translate)
When you travel in Japan, you would have chance to visit a shrine.
This torii is related to Shinto(Japanese religion).
(Shrine is the religious institution of Shinto)
Usually, we see gate in front of the shrine. Every shrine has this torii.
And the shrine is written like this symbol on the map.
http://upload.wikimedia.org/wikipedia/commons/4/4a/Japanese_Map_symbol_%28Shrine%29.svg

What is this for? I am not sure... I guess most ot Japanese people don't know
what is the meaning of this gate... ^^;
There may be some theories. I have not studied well so I can't explain from academic
point of view.
One of them seem to be below.
It is the boundary line between our world and the sacred world.
It is like the entrance.

I would like to introduce the Tamamura hachimangu(the shrine close to my house)
in the future.

Thanks for reading. Please give me any advices.

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