a guide member slidably mounted on said head in {HEAD ~ a direction} parallel to {TAIL ~ the axis} of said measuring element 
parallel to
a guide member slidably mounted on said head in { HEAD ~ a direction } parallel to { TAIL ~ the axis } of said measuring element
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH