The reason the U.S. Army has an Adjutant-General and a Surgeon-General is because during the 19th century they were the only general officers authorized for their respective departments. My Revised U.S. Army Regulations of 1863 makes that point clear in its officer pay tables. The pay tables for the enlisted men of 1863 include the payroll deduction for support of the Old Soldiers Home in Washington DC, just like today.

Old Army publications provide clues about where some of our terminology came from -- in 1863 Boards of Survey were convened to investigate cases of missing Army property, which isn't much different than our current terms of Surveying Officers and Reports of Survey. Endorsements to correspondence are an old 19th-century U.S. Army practice that continue to this day -- see my "Guerillas Near Washington DC" thread in the Historians forum to read endorsements by JEB Stuart and Robert E. Lee to several of Mosby's reports.