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Other Terms: Nervus phrenicus, Nerf phrénique
The phrenic nerve supplies the diaphragm. It arises from the third and fourth cervical nerves and receives a communicating branch from the fifth. It passes downward and inward over the anterior surface of the scalenus anterior muscle, beneath the omohyoid muscle, the transversalis colli and suprascapular arteries, and the left side of the thoracic duct. It enters the upper opening of the chest behind the subclavian vein and in front of the subclavian artery. It then crosses in front of the internal mammary artery. Then it passes inward to the root of the lung. It passes through the middle mediastinum between the mediastinal layer of the pleura and pericardium. It then pierces the diaphragm for its final distribution upon its lower surfaces. At the lower part of the neck, it is joined by a filament of the sympathetic nerve. At times it is joined instead by a branch from the nerve to the subclavius muscle. In the chest, it is accompanied by the pericardiacophrenic artery which is a branch of the internal mammary artery. The origin of the phrenic nerve is mainly from the fourth cervical segment of the spinal cord, which is situated behind the upper part of the body of the fourth cervical vertebra.