THE AUTONOMIC NERVOUS SYSTEM 385 



efferent fibres which come out from the spinal cord in the neck, 

 thorax, and lumbar regions together constitute the sympathetic 

 system ; those which leave the central nervous system in the 

 head from the brain, and from the spinal cord in the sacral 

 region, constitute the parasympathetic system. The word 

 " sympathetic " is sometimes loosely used as synonymous with 

 " autonomic," which introduces confusion. The sympathetic 

 system may be called the " thoracico-lumbar " outflow, and 

 the parasympathetic system the " cranio-sacral outflow." 



The autonomic system may now be described in greater 

 detail, in a typical mammal, and commencing with its sympa- 

 thetic constituent. 



The visceral efferent fibres in the cervical, thoracic and 

 lumbar regions of the spinal cord run out through the ventral 

 roots and down the rami communicantes to the sympathetic 

 ganglia situated on each side of the aorta. These fibres are 

 preganglionic or connectors, and their cell-bodies are in the 

 grey matter of the spinal cord ; they are surrounded by 

 medullary sheaths and these rami communicantes are conse- 

 quently white. 



Some of the preganglionic fibres stop in the sympathetic 

 ganglion corresponding to the segment in which they emerge 

 from the spinal cord, others continue to the next sympathetic 

 ganglia in front or behind and end there. In this way, the 

 sympathetic ganglia of each side become connected together 

 forming the lateral sympathetic chains, and the ganglia on 

 them are called the lateral ganglia. In the region of the neck, 

 several of these lateral ganglia join up close together, forming 

 the large anterior and posterior cervical ganglia and the stellate 

 ganglion. 



Yet other preganglionic fibres run out through the lateral 

 ganglia, but do not stop there. Instead, they run on and end 

 in groups of ganglia situated near the base of the coeliac, 

 anterior and posterior mesenteric arteries. The most im- 

 portant of these ganglia, which are called collateral, are the 

 anterior mesenteric and the posterior mesenteric ganglia. 

 The long rami communicantes which connect these ganglia 

 with the spinal nerves are the splanchnic nerves. 



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