The sympathetic system consists of two long chains of ganglia lying one on each side of the vertebral column, extending from the base of the skull to the coccyx. The ganglia are connected to the spinal cord by white rami communicantes given off by some of the spinal nerves (second thoracic to second lumbar and three or four sacral) as they emerge from the intervertebral foramina. The ganglia send grey rami communicantes back to the cord. Above, the chain ends by a plexus of nerves sent into the cranial cavity on the internal carotid artery, and below, the two terminal ganglia communicate with one another.

The ganglia send branches of supply to vessels, viscera, involuntary muscles, and glands.

In the cervical region there are only three ganglia on each side, but below this there is a ganglion on each side opposite each vertebra.

The ganglia in the cervical and thoracic regions form plexuses on the vessels in their vicinity, and also form plexuses to supply the pharynx, larynx, heart, and lungs. In addition they communicate with the lumbar ganglia by means of the splanchnic nerves, which pierce the diaphragm to join the solar plexus.

The Splanchnic Nerves, three in number, are formed by branches from the fifth to the twelfth thoracic ganglia.

The Solar Plexus consists of three parts-the coeliac plexus and the two semilunar ganglia. They lie behind the stomach, on the aorta, just above the coeliac axis. They send out branches, which form subsidiary plexuses, accompanying the branches of the artery to the various organs.

The Aortic Plexus is the continuation on the front of the aorta of the solar plexus, and, like the latter, forms subsidiary plexuses on the branches of the vessel.