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