This section is from the book "The Constitutional Law Of The United States", by Westel Woodbury Willoughby. Also available from Amazon: Constitutional Law.
There are nine judicial circuits, each circuit being subdivided into districts. In some cases two circuit judges, and in other cases three or more, being appointed for each circuit. One justice of the Supreme 'Court is assigned to each circuit, and as thus assigned is termed circuit justice.
Circuit Courts may be held by the circuit justice, or by a circuit judge of the circuit, or by the district judge of the district, each sitting alone, or by any two of these judges sitting together.5 other district of the circuit within which their districts lie,6 and any one of them may upon the designation of the Chief Justice hold the District and Circuit Court of any District in a Circuit contiguous to his own.