To cut a long story short, the strong outer pane is put there to do the appropriate work, whereas the middle pane is accessible in the case of any kind of emergency or disaster.
Each single window in the airplanes has the same small hole for the safety of the passengers. More formally, it is known as a breather hole and it is involved in regulating the quantity of pressure that passes through the window’s inner as well as outer panes. In a word, the system makes sure that the outer pane tolerates the largest part of the pressure with the intention that if there were any situation that becomes reason of extra stress on the window of the airplane, it is the outside pane that removes out that simply meaning you are able to breathe till now.
The small breather hole also helps in keeping the window fog free by wicking moisture which gets caught between the window panes of the airplane.
To cruise at the height of 10,600 meters (35,000 feet), the atmospheric pressure is nearly around 1.5 kilograms (3.3 pounds) for each square inch. At such a height under such pressure it is too difficult for the human bodies to stay alert as well as conscious, thus the atmospheric pressure is artificially sustained in the region of 3.5 kilograms (8 pounds) for each square inch. But obviously, if the air pressure inside is increased, the structure of the panes has to be sturdy and strong to hold the variation between the air pressure outside the plane and air pressure inside the plane.
The whole thing done to the airplane is just meant for its safety and security. Every single step that is taken in structuring a plane is to avoid any mishap during the flight. To avoid any kind of breathe problem the small breathe hole is kept there in window panes to keep passengers breath easily. Moreover the hole reduces the fog that usually stuck on the window.
The aircraft windows have multiple layers which are thick and broad. The side which faces the passengers is merely a sheet of plastic, as they would not wish for the passengers scraping up the actual expensive and costly window of glass. Away from the inner pane of the window there exist two definite structural window panes. The outer pane is generally the one that bears the load, whereas the inner pane works as a backup. Though, since air pressure varies in the cabin of the plane during the flight, it is also essential to make equal the air pressure between the window panes, and thus the small hole. A hole of small size would not put at risk pressurization even though the outer pane binged. The whole thing is meant to avoid any disaster.
Now the mystery is revealed before you. Hopefully, whenever you have a flight for the next time, you will surely look for the window and share your findings with your fellow beings.