The Phrenic Nerve (n. phrenicus; internal respiratory nerve of Bell) contains motor and sensory fibers in the proportion of about two to one.
It arises chiefly from the fourth cervical nerve, but receives a branch from the third and another from the fifth (accessory phrenic nerves); (the fibers from the fifth occasionally come through the nerve to the Subclavius).
It descends to the root of the neck, running obliquely across the front of the Scalenus anterior, and beneath the Sternocleidomastoideus, the inferior belly of the Omohyoideus, and the transverse cervical and transverse scapular vessels. It next passes in front of the first part of the subclavian artery, between it and the subclavian vein, and, as it enters the thorax, crosses the internal mammary artery near its origin.
Within the thorax, it descends nearly vertically in front of the root of the lung, and then between the pericardium and the mediastinal pleura, to the diaphragm, where it divides into branches, which pierce that muscle, and are distributed to its under surface. In the thorax it is accompanied by the pericardiacophrenic branch of the internal mammary artery.
The two phrenic nerves differ in their length, and also in their relations at the upper part of the thorax :
Each nerve supplies filaments to the pericardium (Pericardial branch) and pleura, and at the root of the neck is joined by a filament from the sympathetic, and, occasionally, by one from the ansa cervicalis. Branches have been described as passing to the peritoneum.
From the right nerve, one or two filaments pass to join in a small phrenic ganglion with phrenic branches of the celiac plexus; and branches from this ganglion are distributed to the falciform and coronary ligaments of the liver, the suprarenal gland, inferior vena cava, and right atrium. From the left nerve, filaments pass to join the phrenic branches of the celiac plexus, but without any ganglionic enlargement; and a twig is distributed to the left suprarenal gland. These fibers for peritoneum and abdomen are termed phrenicoabdominal branches.