First, since {TAIL ~ the substrates} 16 are removeably held in {HEAD ~ the first housing portion} 28, it is very convenient to replace any one of the substrates 16 with another substrate for a different function 
First , since { TAIL ~ the substrates } 16 are removeably held in { HEAD ~ the first housing portion } 28 , it is very convenient to replace any one of the substrates 16 with another substrate for a different function
OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH