Your breathing muscle sits under the ribs but is wired from the neck
The diaphragm lies at the bottom of the chest, yet its nerve supply leaves the spinal cord high in the neck. The reason is embryological: the tissue that becomes its central tendon first forms near the head and migrates downward, dragging the phrenic nerve on a long detour around the heart.
The diaphragm is a dome of skeletal muscle and fibrous tissue that divides the chest, holding the heart and lungs, from the abdomen. Its name comes from the Greek for partition, and the anatomical term was coined by Gerard of Cremona. It is the chief muscle of breathing, and it is packed with more mitochondria and capillaries than any other skeletal muscle, a sign of how much oxygen it burns. In people it is slightly lopsided: the right side rides higher because the bulky liver sits beneath it.
When it contracts, the dome flattens and moves down, enlarging the chest and lowering the pressure inside so that air flows into the lungs. Relaxing lets the elastic lungs recoil and push air out; forceful exhaling recruits the abdominal and internal intercostal muscles. Structurally it has two regions. The costal part does the pulling, while the crural part anchors the muscle to the lower ribs and lumbar vertebrae. All its fibres converge on a central tendon at the crest of the dome.
Three large openings let traffic pass between chest and belly: one each for the oesophagus, the aorta and the inferior vena cava. The aorta actually slips behind the muscle between its two crura rather than piercing it. The phrenic nerve, built from spinal nerves C3, C4 and C5, drives the muscle. Its origin traces back to the third week after fertilisation, when the septum transversum, the forerunner of the central tendon, starts out in the neck region before descending.
The muscle also raises abdominal pressure to help expel waste, assists childbirth and squeezes the oesophagus to resist acid reflux. Problems arise when that barrier fails. In a hiatus hernia, part of the stomach or lower oesophagus slides up into the chest, which is linked to reflux. A congenital diaphragmatic hernia, seen in roughly 0.8 to 5 of every 10,000 births, lets abdominal organs crowd the developing lungs and needs urgent surgery when large.
Source: Thoracic diaphragm
