its the flowmeter that delivers the nitrous or oxygen into the mixing chamber ,
after this mixing it enters into the "bag-tee" fititng on the machine,
as the gas flows through the bag-tee, it takes a short trip into the reservoir bag which is attached to the bag-tee tube at its bottom , so that the gas flow will be even without any swings in their concentrations...
hope this helps
flowmeter, reservoir bag, bag-tee were previously asked in NDBE part 2...