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