At the base, the lumens are approximately circular in cross section, whereas at the distal ends, {HEAD ~ the lumens} are approximately rectangular in {TAIL ~ cross section} 
are approximately rectangular in
At the base , the lumens are approximately circular in cross section , whereas at the distal ends , { HEAD ~ the lumens } are approximately rectangular in { TAIL ~ cross section }
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 REL REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL