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