It consists of an array of plates, serrated on an edge, and assembled in a block whose midline is continuous with {HEAD ~ the entry} to {TAIL ~ the fixed guide} 
to
It consists of an array of plates , serrated on an edge , and assembled in a block whose midline is continuous with { HEAD ~ the entry } to { TAIL ~ the fixed guide }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL