As such, {HEAD ~ the main body} 12, by itself, is essentially {TAIL ~ a conduit} through which material may pass 
is essentially
As such , { HEAD ~ the main body } 12 , by itself , is essentially { TAIL ~ a conduit } through which material may pass
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH