Cut-away illustration of the chest showing the phrenic nerves (yellow). The left and right phrenic nerves run from the top of the neck to the thoracic diaphragm. The diaphragm is a sheet of muscle that stretches across the bottom of the ribcage separating the thoracic (chest) cavity from the abdominal cavity. The diaphragm contracts and relaxes during inhalation and exhalation. This is an involuntary movement that is transmitted from the brain through the phrenic nerves. From Traite d'Anatomie Humaine, Vol 3 (1905) by French anatomist Leo Testut (1849-1925).