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