The rear wall of the fourth case is in communication with the other of the side walls of each of {HEAD ~ the first case} and {TAIL ~ the second case} 
The rear wall of the fourth case is in communication with the other of the side walls of each of { HEAD ~ the first case } and { TAIL ~ the second case }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL