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