The chief muscles of expiration are the four sets of abdominal muscles and the intercostal muscles that depress the ribs.

The diaphragm is _not_ a muscle of _expiration_.
THE DIAPHRAGM The diaphragm is in form like an inverted bowl (Fig.

8).

It forms the floor of the thorax (chest) and the roof of the abdomen.

It is attached by a strong tendon to the spinal column behind, and to the walls of the thorax at its lowest part, which is below the ribs.