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