The Senate regularly receives messages from the president of the United States and the House of Representatives. When a messenger enters the Chamber at the rear of the center aisle facing the presiding officer, that officer suspends the proceedings to recognize the emissary, who hands the message to a Senate aide, and departs.
When the Senate first convened in 1789, it spent considerable emotional energy on protocol issues. Driving the Senate's concern was a desire to ensure its equal–if not perhaps superior–status relative to the House. This was evident in the Senate's plan for delivering messages and legislation between the two chambers. The Senate considered its secretary a suitable envoy to the House. For House-passed legislation, however, the Senate desired that two members of that lower chamber serve as messengers. The House laughed at the Senate's proposal and instituted the tradition–parallel to that of the Senate–of sending all items under the supervision of an elected officer.
Baker, Richard A. The New Members' Guide to Traditions of the United States Senate. (Washington, GPO, 2006. S.Pub. 109-25), 20.