{HEAD ~ The cavity} has generally vertical side walls 13 and 14, end walls 15 and 16 and {TAIL ~ a generally rectangular floor} 17 
has
{ HEAD ~ The cavity } has generally vertical side walls 13 and 14 , end walls 15 and 16 and { TAIL ~ a generally rectangular floor } 17
HEAD HEAD HEAD HEAD HEAD HEAD REL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH