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