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