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