{HEAD ~ a vacuum source} fluidly connected to {TAIL ~ the scavenging tube} for scavenging excess gases; and 
fluidly connected to
{ HEAD ~ a vacuum source } fluidly connected to { TAIL ~ the scavenging tube } for scavenging excess gases ; and
HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH