The rear wall of the second case is in {HEAD ~ communication} with {TAIL ~ the rear wall} of the first case 
The rear wall of the second case is in { HEAD ~ communication } with { TAIL ~ the rear wall } of the first case
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH