Each of the casings includes a coverable opening through which {HEAD ~ the electrolyte solution} within {TAIL ~ the cell} is replaceable 
within
Each of the casings includes a coverable opening through which { HEAD ~ the electrolyte solution } within { TAIL ~ the cell } is replaceable
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH