The answers to [this question][1] indicate that these are the exhaust vents for the cabin air circulation. I think they appear to be much too large for just that purpose. I don't have a technical reference for this, but I believe those vents also prevent the cabin floor buckling in the event of a sudden pressurization failure.

The cargo hold is beneath the passenger cabin, and both are pressurized because the cylindrical hull can resist pressure better than the flat cabin floor. If the cabin suddenly loses pressure the air in the hold could buckle the floor upward, or if the hold loses pressure the air in the cabin could buckle the floor downward. The floor must be generously vented to prevent this.

If these really are only for cabin circulation, I have to wonder -- where are the pressure differential relief vents?




  [1]: http://aviation.stackexchange.com/questions/1672/how-effective-are-modern-aircraft-at-clearing-smoke