Cervical PlexusThe cervical plexus is formed by the anterior primary divisions of the first four cervical nerves. The nerves emerge from the intervertebral foramina behind the vertebral artery, and each nerve is joined at once by a communicating branch from the sympathetic ganglion. The plexus lies on the scalenus medius muscle, and is covered by sterno-mastoid. The four nerves join with another, forming a series of loops, from which the branches of distribution arise.Branches(1) Cutaneous to head, neck, and shoulder.(2) Muscular to muscles of neck and diaphragm.(3) Communicating to vagus, spinal accessory, hypoglossal, and sympathetic.(1) The Cutaneous branches appear in the posterior triangle of the neck. The three ascending ones turn upwards over posterior border of sterno-mastoid, the three descending ones to the clavicular region.Ascending Branches: (a) Small occipital supplies skin on back of ear and on scalp over mastoid process.(b) Great auricular crosses sterno-mastoid obliquely upwards, and supplies the scalp behind the ear, the lower part of the pinna, and the skin over the lower part of the masseter and the parotid gland.(c) Superficial cervical crosses straight over the sterno-mastoid and supplies the skin over the anterior triangle of the neck.Descending Branches: One large trunk is formed which descends from beneath sterno-mastoid. It extends through the posterior triangle of the neck and supplies the skin over the clavicle by dividing into three parts-sternal, clavicular, and acromial.(2) The Muscular branches of the plexus are in two sets-internal and external, according to their relation to the sterno-mastoid muscle.External branches passing outwards to posterior triangle supply sterno-mastoid, levator scapulae, trapezius and the scaleni, and communicate with the spinal accessory nerve.

Fig. 63 - Nerves in Posterior Triangle of Neck.Internal branches passing inwards to anterior triangle supply the prevertebral muscles, genio-hyoid, and infrahyoid muscles, and communicate with the vagus and hypoglossal. There is a special branch to the diaphragm called the phrenic nerve.Phrenic nerveThe Phrenic nerve is formed by branches from the third, fourth, and fifth cervical nerves. It passes down on scalenus anticus, and enters the thorax between the subclavian artery and vein. It reaches the diaphragm by passing between the pericardium and pleura in front of the root of the lung. The nerve gives off some branches to the upper surface of the diaphragm, then pierces the muscle and supplies the under surface. It also gives off pleural, pericardial, and hepatic branches

Fig. 64. - Vessels and Nerves in Neck.