In that position, {HEAD ~ the vent} is located at {TAIL ~ the interface} of the two cells 
is located at
In that position , { HEAD ~ the vent } is located at { TAIL ~ the interface } of the two cells
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH