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