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