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