The phrenic nerve on both sides originates from the ventral rami of the third to fifth cervical nerves. It passes inferiorly down the neck to the lateral border of scalenus anterior. Then, it passes medially across the border of scalenus anterior parallel to the internal jugular vein which lies inferomedially. At this point it is deep to the prevertebral fascia, the transverse cervical artery and the suprascapular artery.
At the anteroinferior medial margin of scalenus anterior, the left phrenic nerve crosses the first part of the left subclavian artery and then the internal thoracic artery sited slightly inferiorly. Passing inferiorly with the internal thoracic artery laterally, it lies deep to the left brachiocephalic vein and the left first costochondral joint. It receives a pericardiophrenic branch of the internal thoracic artery which stays with its distal course.
Within the thorax, the phrenic nerve continues inferiorly and slightly laterally on the anterolateral aspect of the arch of the aorta, separated from the posterior right vagus nerve by the left superior intercostal vein.
Then it descends anterior to the root of the left lung intermediate to fibrous pericardium medially and parietal pleura laterally. Finally, it curves inferiorly and anteriorly to reach the surface of the diaphragm which it pierces anterior to the central tendon and lateral to the pericardium. It then forms three branches on the inferior surface of the diaphragm: anterior, lateral and posterior. These ramify out in a radial manner from the point of perforation to supply all but the periphery of the muscle.
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