{HEAD ~ The gas} may be passed through the cavity in the double wall and may be expelled from {TAIL ~ the cavity} 
expelled from
{ HEAD ~ The gas } may be passed through the cavity in the double wall and may be expelled from { TAIL ~ the cavity }
HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH REL REL TAIL TAIL TAIL TAIL TAIL TAIL