THE PHRENIC NERVE:
Netter Plate Number: 28A

The phrenic nerve is the motor nerve to the diaphragm, (and some sensory components to the central lung).
The nerve arises mainly from a large root from C4, with contributions from C3 and C5.
It appears along the lateral portion of the anterior scalene muscle over the ventral surface, where it enters the chest along the medial border. The nerve is contained within the prevertebral layer of cervical fascia.
This nerve parallels the path of the ascending cervical artery, and is crossed by the transverse cervical artery and then the suprascapular artery respectively as it descends down the anterior scalene.

Occasionally components from C5 can travel and penetrate the subclavius muscle to join the phrenic inferiorly in the chest.
This is clinically significant, as if the phrenic is cut to paralyze the diaphragm, these ancillary fibers may continue to innervate the diaphragm.