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