Diaphragm

The diaphragm is the dome-shaped sheet of muscle that separates the chest from the abdomen. It is attached to the spine, ribs and sternum and plays a very important role in the breathing process. The lungs are enclosed in a kind of cage in which the ribs form the sides and the diaphragm, an upwardly arching sheet of muscle, forms the floor. When we breathe, the diaphragm is drawn downward until it is flat. At the same time, the muscles around the ribs pull them up like a hoop skirt. The chest cavity becomes deeper and larger, making more air space. The muscle fibers of the diaphragm converge on the central tendon, which is a thick, flat plate of dense fibers. There are openings in the diaphragm for the esophagus, the phrenic nerve (which controls the movements of the diaphragm to produce breathing), and the aorta and vena cava blood vessels, which lead to and from the heart. When air is drawn into the lungs, the muscles in the diaphragm contract, pulling the central tendon down. This enlarges the chest, and air then passes into the lungs to fill the larger space. The diaphragm sometimes contracts involuntarily because the controlling nerves are irritated by eating too fast (or for some other reason). At this time, if air is inhaled, the space between the vocal cords at the back of the throat close suddenly, producing the clicking noise we call "hiccups."