In an equilibrium position, {HEAD ~ the vent} is closed off by {TAIL ~ the vent follower} 
is closed off by
In an equilibrium position , { HEAD ~ the vent } is closed off by { TAIL ~ the vent follower }
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL