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