Where does the air enter the passenger cabin? After passing through the packs and turbines, where does the air get into the cabin? On the 737-800, for example. I don't see on the roof any grill to let the air enter the cabin, look:


Source: Airliners.net

Where does the air enter the cabin? According this graphic, the air enters through the roof and exits through the wall:
It looks to me in your top image that the air vent is right there at the center of the ceiling. 



It looks like a linear slot diffuser running the length of the cabin.  Similar to this