{HEAD ~ The mid wall} has {TAIL ~ a longitudinally extending upper boss} and a longitudinally extending lower boss forming an outwardly open slot therebetween facing the outer wall 
has
{ HEAD ~ The mid wall } has { TAIL ~ a longitudinally extending upper boss } and a longitudinally extending lower boss forming an outwardly open slot therebetween facing the outer wall
HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH