Preferably, {HEAD ~ the projection} extends inwardly of {TAIL ~ the structure} and is of circular cross section 
Preferably , { HEAD ~ the projection } extends inwardly of { TAIL ~ the structure } and is of circular cross section
OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH