What is this in the window? Every time I have flown and sat in a window seat, I have noticed this small tube at the base of the window. 



It looks like it might be either metal or a small air pocket, but I'm not sure. I've been seeing it for years and always wondered what it is.

What is this "tube" embedded into airline windows, and does it have any purpose?
As pressurized aircraft's windows exist of several layers (usually three) there's this little hole to compensate differences in the pressure between inside and outside the aircraft.
Inside the aircraft the pressure is much higher than outside so without this little hole the windowd layers would be pushed together and break.