The contraction of spring 20 causes {HEAD ~ needle} 14 to be retracted into {TAIL ~ the void space} 15 in sensor base unit 18 
to be retracted into
The contraction of spring 20 causes { HEAD ~ needle } 14 to be retracted into { TAIL ~ the void space } 15 in sensor base unit 18
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH REL REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH