{HEAD ~ The block} usually includes {TAIL ~ a plurality} of interconnected gas containing tunnels or passages which form a closed loop path in the shape of a triangle, a rectangle, or any polygonal path 
usually includes
{ HEAD ~ The block } usually includes { TAIL ~ a plurality } of interconnected gas containing tunnels or passages which form a closed loop path in the shape of a triangle , a rectangle , or any polygonal path
HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH