More preferably, {HEAD ~ the duct} is defined by {TAIL ~ a groove} in an outer surface of the hub 
is defined by
More preferably , { HEAD ~ the duct } is defined by { TAIL ~ a groove } in an outer surface of the hub
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH