THE SYMPATHETIC NERVOUS SYSTEM 



337 



the passage of impulses through the sympathetic ganglia, although an undi- 

 minished reaction may be obtained by stimulation of the more peripheral sym- 

 pathetic nerves By a long series of experiments Langley has shown that there 

 are always two and probably never more than two neurons concerned in the 

 conduction of an impulse from the central nervous system to smooth muscle 

 or glandular tissue. The neurons of the first order in this series are designated as 

 preganglionic, those of the second order as postganglionic, with reference to the 

 relation which they bear to the ganglion containing their synapse. 



Preganglionic neurons have their cell bodies located in the visceral efferent 

 column of the cerebrospinal axis. The cells of this series are smaller than those 



Spinal ganglion 

 Dorsal ramus 



,' Ventral ramus 



Ramus communicans 



--- Sympathetic ganglion 



<y\ Visceral efferent fiber 

 Somatic efferent fiber 



<^1 Postganglionic fiber 



root 



' ______ ,Viscus 



Fig. 249. Diagrammatic section through a spinal nerve and the spinal cord in the thoracic region 

 to illustrate the chief functional types of peripheral nerve-fibers. 



of the somatic motor column and contain less massive Nissl granules. From 

 these cells arise the fine myelinated visceral efferent fibers which run through 

 the cerebrospinal nerves to the sympathetic nervous system and terminate in 

 the sympathetic ganglia (Fig. 249) . 



Postganglionic neurons have their cell bodies located in the sympathetic 

 ganglia. In fact, these cells with their dendritic ramifications and the terminal 

 branches of the preganglionic fibers synaptically related to them are the es- 

 sential elements in the sympathetic ganglia. Their axons for the most part 

 remain unmyelinated and run as Remak fibers through the sympathetic nerves 



