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