This is the portion of the nervous system by which the viscera are principally supplied. The primary part consists of two chains of ganglia, one on each side, in front of the vertebral column, called the prevertebral chains, or the great sympathetic.

In the dorsal, lumbar, and sacral regions, these chains presents, ganglion for almost every spinal nerve, and each spinal nerve has a twig of communication with its corresponding ganglion. Interiorly the two chains meet together in a ganglion impar in front of the coccyx. In the neck there axe only three ganglia, but they communicate with all the cervical nerves; and the uppermost ganglion, which is the largest, sends branches upwards into the skull, round the internal carotid artery. Within the skull the chain can be traced, in somewhat irregular fashion, communicating with the fifth and other nerves; and two cords pass forwards, one on each side of the septum of the nose, to unite on the palate behind the incisor teeth, and form the superior termination of the chain, at the spot which I once demonstrated, and still hold, to be the arch of the foremost segment of the skull.

In the neck, the sympathetic chains give off branches to the heart; and in the chest they send twigs to the lungs. From the thoracic ganglia there likewise descend three pairs of splanchnic nerves, which form a very large plexus in the upper part of the abdomen, the solar plexus. This plexus contains two large semilunar ganglia, and sends branches along the blood-vessels to the stomach, liver, intestines, and other abdominal viscera, and communicates by large branches on the aorta with the hypogastric plexus, which is placed within the pelvis and supplies the viscera there.

The sympathetic system is especially devoted to the supply of the viscera and blood-vessels, but it is by no means independent of the cerebro-spinal system; as is proved anatomically by its close connection with the pneumogastric nerves and its communications with the spinal cord, and physiologically by the conveyance of influences through it from the cerebro-spinal axis. That nervous influence is so conveyed is illustrated by many familiar effects of mental conditions on different visceral actions, but still more explicitly by the effects of experiments on the nerves to the bloodvessels.