{TAIL ~ The caudad end} contains {HEAD ~ a pressure sensitive passive or preferably, hydraulically or electrically activated, unidirectional valve} 
{ TAIL ~ The caudad end } contains { HEAD ~ a pressure sensitive passive or preferably , hydraulically or electrically activated , unidirectional valve }
TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD