Breathing
Accessory muscles are muscles that a person uses only when they need extra help breathing.  Sometimes this is normal.  For example, if a person has just done a lot of exercise, they may need extra oxygen.  The medulla will tell the accessory muscles to kick in, to make it easier for the person to lift their chest to create more room for the lungs to fill.  The most important accessory muscles are the muscles in the chest, abdomen, and neck.