The sympathetic system consists of (a) two gangliated nerve cords extending along the front of the vertebral column from the basis cranii to the coccyx; (b) several plexuses associated with the cords; and (c) efferent nerves from the cords and plexuses. Each cord comprises twenty-one or twenty-two ganglia of a reddish-grey colour united by vertically-arranged commissural fibres. There are three cervical ganglia, ten or eleven thoracic, four lumbar, and four sacral. These ganglia are termed vertebral. Ganglia are also present in the pre-vertebral plexuses, the collateral ganglia, likewise in the walls of viscera and blood-vessels, terminal ganglia.
The sympathetic effects communication with the anterior primary divisions of the spinal nerves by means of medullated fibres, the white rami communicantes, and non-medullated fibres, the grey rami commumcantes. Each grey ramus gives off a small filament to join the recurrent branch of the corresponding spinal nerve. Of the white rami, some terminate in the vertebral ganglia, while others travel up or down the gangliated cord to emerge as efferent nerves. These may pass to the collateral and terminal ganglia, or proceed direct to the viscera and blood-vessels, The commissural fibres connecting the vertebral ganglia are partly medullated and partly non-medullated, the former being derived from the spinal nerves, the latter from the ganglia themselves,
The cervical portion of the sympathetic is found behind the carotid sheath, resting upon the prevertebral fascia and muscles. It does not possess any white rami communicantes. Three ganglia are usually present, superior, middle, and inferior; the middle ganglion may be absent,