THE SYMPATHETIC NERVOUS SYSTEM 231 



The portions of the sympathetic system related to these respec- 

 tive regions are as follows: (1) The midbrain sympathetic, com- 

 prising chiefly the ciliary ganglion behind the eye and its nerves, 

 these being related to the brain through the III cranial nerve. 

 (2) The bulbar sympathetic, related to the brain chiefly through 

 the VII, IX, and X cranial nerves. (3) The thoracic-lumbar 

 sympathetic, related to the spinal cord through the I thoracic 

 to II or III lumbar nerves. (4) The sacral sympathetic, 

 related to the spinal cord through the II to IV sacral nerves. 



Each of these four regions has its own distinctive physiological 

 characteristics, including in some cases a special type of reaction 

 to certain drugs. They all exhibit a common reaction to nico- 

 tin in physiological doses. The effect of this poison is to paralyze 

 the synapses between the preganglionic and the postganglionic 

 neurons and thus to isolate the peripheral sympathetic neurons 

 physiologically from efferent impulses arising within the central 

 nervous system. Adrenalin (extract of the suprarenal glands) 

 affects chiefly the thoracic-lumbar sympathetic system (see p. 

 255). On the other hand, poisons of a different group, including 

 atropin, muscarin, and pilocarpin, are said to act chiefly upon 

 the midbrain, bulbar and sacral sympathetic, but not upon the 

 thoracic-lumbar system. There are other cases of very specific 

 action of drugs upon special parts of the sympathetic nervous 

 system. 



Summary. From the preceding considerations it is evident 

 that the sympathetic nervous system cannot be sharply sepa- 

 rated anatomically or physiologically from the cerebro-spinal 

 system. The cell bodies of the neurons of the cerebro-spinal 

 visceral system lie partly within and partly without the central 

 nervous axis. A ganglionic sympathetic trunk extends on each 

 side of the body along the spinal column, and the ganglia of this 

 trunk are connected with most of the spinal nerves by com- 

 municating branches. The neurons of this trunk of vertebral 

 sympathetic ganglia belong chiefly to the cerebro-spinal visceral 

 system, since they are concerned with the central regulatory 

 mechanism of the viscera. All parts of the visceral nervous sys- 

 tem which lie peripherally of the communicating branches 

 between the sympathetic ganglionated trunks and the spinal 

 roots, and can be anatomically separated from the peripheral 



