The phrenic nerve is the most important branch of the cervical plexus, as it is the motor nerve of the diaphragm. It enters the thorax behind the subclavian vein ; in passing down it gives branches to the pleurae and pericardium ; finally it breaks up into several branches which pierce the diaphragm and spread over its under surface.

The left pneumogastric (or vagus) nerve enters the thorax between the carotid and left subclavian arteries; behind the root of the left lung it breaks up into a number of branches, which form the left pulmonary plexus, two cords of which go to the front of the oesophagus and form the plexus gulae ; it then passes out of the thorax by the oesophageal opening in the diaphragm, and breaks up into branches on the anterior surface of the stomach.

The right pneumogastric lies deeper in the thorax than the left. It descends by the side of the trachea to the back of the root of the right lung, and breaks up into the right pulmonary plexus, two cords of which assist in forming the plexus gulae ; it leaves the thorax by the same opening as the left, and breaks up into branches on the posterior aspect of the stomach.

The cardiac plexus of the sympathetic system is situated on the lower end of the trachea and behind the aortic arch ; the nerves which form it are from the cervical ganglia of the sympathetic, from the pneumogastric and from the recurrent laryngeal nerves. The thoracic ganglia of the sympathetic are eleven in number ; for the most part they lie on the heads of the ribs. They are continued from the cervical and into the abdominal ganglia.

(For the Splanchnic Nerves, see Chapter V.)

The intercostal or anterior branches of the dorsal nerves are distributed to the chest and abdomen. They do not unite in plexuses ; each one runs separately in a groove in the lower margin of the corresponding rib, and has muscular and cutaneous branches. The dorsal or posterior branches supply the skin and muscles of the back.