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