diaphragm

diaphragm The term has come to be applied to any thin partition, either in a man-made instrument or in biology. But the original meaning from the Greek referred more to hedging around or fencing in; that meaning is well fitted by the contraceptive diaphragm: a thin rubber cup which fences in the neck of the womb against invasion by sperm.

The human anatomical diaphragm is a part muscle, part tendinous sheet, convex upwards, that separates the thorax from the abdomen. It is described as doubly ‘domed’, accommodating below it the liver on the right and the stomach and spleen on the left. It is attached to the inside of the lower margin of the ‘rib cage’: from the sternum (breast bone) in front, the cartilages interposed between this and the lower ribs around the sides, to reach the vertebral column behind. It is a complete partition except for openings to allow passage between thorax and abdomen: for the oesophagus to join the stomach, for the main artery (aorta) and for the main vein (inferior vena cava) that carry blood to and from the lower body, and for nerves and lymphatics vessels.

When the muscle of the diaphragm contracts, its convexity is flattened. This, together with other mechanical factors and muscular actions, causes an increase in the capacity of the thorax, which in turn draws air into the lungs. The motor nerve supply to the diaphragm comes to its upper surface in the phrenic nerves from each side of the neck. These carry the rhythmic bursts of impulses, relayed from the brain stem via the phrenic motor neurons in the spinal cord, which cause regular inspiration. Each time these stop, the diaphragm relaxes and rises, and air leaves the lungs. Although this cyclical control of the diaphragm and other breathing muscles occurs automatically, it is of course possible to control them voluntarily — to stop, start, or deepen breathing. The extent to which the diaphragm is used relative to the muscles of the thorax can also come under voluntary control, by selecting ‘abdominal’ breathing rather than chest expansion.

Sheila Jennett

See respiratory system.See also breathing.