 FACT: A 737 has three hydraulic systems, System "A" drives the  primary flight controls and landing gear, system "B" drives the  flaps and brakes and is the backup for system "A", system "C" is the  backup for the rudder and flaps in the event "A" and "B" are inop.  The ailerons and elevators are mechanically linked to the yoke using  traditional cables and pulleys, the hydraulics simply augment the  input forces so the flight crew doesnt have to work as hard. So,  even if the lost all hydraulic fluid they would still have control  using manual reversion in the control linkages (except rudder), also  the undercarriage can be manually lowered.