Medical Service to Field Units — FM 8-10

The medical service of the United States Army is organized into five functional echelons. These are, from front to rear: unit medical service, division medical service, army medical service, and the medical services, respectively, of the theater of operations and of the zone of the interior. The first three of these echelons serve mobile tactical units and are, therefore, made up exclusively of mobile medical units. The last two echelons pertain to territorial commands and the medical installations thereof are, for the most part, fixed.