The part of the autonomic nervous system which chiefly contains adrenergic fibres and tends to control and reduce secretions, decrease the contractility and hence the tone of smooth muscles and provoke the contraction of blood vessels. Essentially, it consists of preganglionic fibres from the thoracic and upper lumbar parts of the spinal cord. These fibres, by means of delicate rami communicantes, cross over to ganglia sited in a pair of ganglionated cords on each side of the vertebral column or to more peripheral ganglia.
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