This plexus is formed by the upper four cervical nerves. The lower three each divide into an ascending and descending branch, which, joining together, form the plexus. It is opposite the first four upper vertebrae, beneath the upper part of the sterno-mastoid muscle, and in front of the scalenus medius and levator anguli scapulae muscles. Its branches are numerous ; a superficial set supply the cervical fascia, the skin of the head and neck, and part of the ear and face ; a deep set are distributed to the muscles of the neck and the upper part of the back. The most important nerve of this plexus is the phrenic ; it arises mainly from the fourth cervical nerve, and enters the thorax behind the subclavian vein, and proceeds down to the diaphragm, where it breaks into several branches, which pierce the muscle and spread out on its under surface to supply it. The diaphragm is the great muscle of respiration, and any injury to the phrenic nerve or to the spinal cord above the origin of its fibres causes paralysis of the diaphragm and immediate death.

Fig. 27.—Cervical and Brachial Plexus.

1, Musculo-cutaneous; 2, musculo-spiral; 3, median; 4, ulnar; 5, phrenic nerve ; 6. outer cord ; 7, inner cord ; 8, posterior cord.