At each intersection of {HEAD ~ a pair} of {TAIL ~ interconnected tunnels} is a mirror mounted on the block 
of
At each intersection of { HEAD ~ a pair } of { TAIL ~ interconnected tunnels } is a mirror mounted on the block
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH