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