In {TAIL ~ the central area}, {HEAD ~ the two longitudinal wall parts} 14 and 15 have central supports 20, 20′, etc 
In
In { TAIL ~ the central area } , { HEAD ~ the two longitudinal wall parts } 14 and 15 have central supports 20 , 20′ , etc
REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH