Here is an example of an atmospheric gas burner and its major parts. An orifice in the gas manifold directs and regulates the flow of gas into the burner. The velocity of the gas flow creates a venturi effect that draws in primary air to mix with the gas before ignition. Most of these burners have air shutters at the inlet to control the flow of primary air.
Excerpt from the Instructor Series: Preparing for the NATE Gas and Oil Heating Exam, a Powerpoint slide series. Available through the online store at the Refrigeration Service Engineers Society. RSES Online Store