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