The autonomic nervous system contributes through its various actions to the homeostasis of the organism, that is to the maintenance of the optimal internal environment of the body. It fulfils this function by a fine control of smooth muscles, cardiac muscle, exocrine glands and some endocrine glands. Anatomically it is composed of two divisions: the sympathetic and the parasympathetic systems (Fig. 18.1). The sympathetic fibres stem from cells situated in the thoracic and two upper lumbar segments, while the parasympathetic fibres have cells in the brain stem and in the second to fourth sacral segments of the spinal cord. Because of this cellular localisation, the sympathetic system is also known as the thoracolumbar and the parasympathetic system as the craniosacral.
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© D. Ottoson 1983