From the gangliated cord which lies at each side of the spinal column an external and internal series of branches are given off.

FIG 29.—Chain of Sympathetic System (Right Side).

By the external branches the ganglia are connected with the spinal and cranial nerves in their respective neighbourhood. Some fibres pass from the ganglia to the spinal nerves ; others pass from the spinal nerves to the ganglia. Fibres from the cervical ganglia join the cranial nerves ; thus the whole nervous system is united.

The internal branches are distributed to the viscera and to the walls of the bloodvessels. In the viscera they are joined by branches from the cerebro-spinal system, and form plexuses about the heart, lungs, stomach, and other organs, and to a certain extent control their functions. In the bloodvessels they regulate the supply of blood to the various parts of the body. Vaso-motor nerves are of two kinds—vaso-constrictors and vaso-dilators. Under the influence of the former the arterial walls are caused to contract, and the flow of the blood through them is retarded. This explains the act of turning pale as the result of some emotion. The action of the vasodilators is the reverse of this ; the influence of some emotion is extended to the sympathetic system, and causes dilation of the arteries, and a greater quantity of blood is allowed to pass. This explains the act of blushing.

The most important of the sympathetic nerves are the splanchnic. There are three.

The great splanchnic nerve is formed by branches from the sixth to the tenth ganglia of the cord, and ends chiefly in the semilunar ganglia of the solar plexus.

The small splanchnic nerve is formed by branches from the tenth and eleventh ganglia of the cord, and ends in the cœliac plexus.

The smallest splanchnic arises from the twelfth ganglion, and ends in the renal plexus. In connection with the sympathetic system there are three large plexuses in front of the spinal column, called the pre-vertebral plexuses. They are the

Cardiac,

Solar or epigastric,

Hypogastric.

The cardiac plexus is behind the aortic arch upon the lower end of the trachea. It is formed by the interlacement of the cardiac branches of the sympathetic and pneumogastric nerves.

The solar plexus is behind the stomach, in front of the aorta and pillars of the diaphragm ; it is the largest of the three, and is formed by branches of the pneumogastric and great splanchnic nerves. On each side of it there is a ganglion called the semilunar, the upper part of which is joined by the great splanchnic nerve, and the lower part by the small splanchnic nerve. The two semilunar ganglia are joined by the coeliac plexus, and other secondary plexuses arise from the coeliac.

The hypogastric plexus is in front of the body of the last lumbar vertebra, between the two common iliac arteries. It is formed by branches of the aortic plexus and lumbar ganglia. The pelvic viscera is mainly supplied with nerves from it. At the lower part it divides into two parts, which go down the sides of the rectum and join other small plexuses.