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