CHAPTER XXI. 



THE SYMPATHETIC NERVOUS SYSTEM. 



HAYING in the preceding chapters completed the description of the 

 Cerebro-spinal nervous system, there remains to be considered the struc- 

 ture and functions of the so-called Sympathetic nervous system, and to 

 this it is now necessary to direct attention. 



It should, however, be borne in mind that the cerebro-spinal and 

 sympathetic systems are so very intimately connected that the separation 

 of the one from the other may be considered to be purely for the sake 

 of convenience. 



Distribution. The various ganglia and nerves of which the sympa- 

 thetic system is generally said to consist have been already enumerated. 

 Gaskell's researches have suggested a convenient classification of the 

 former into: (1.) The main sympathetic chain, extending from above 

 downwards, in the form of connected ganglia lying upon the bodies of 

 the vertebrae, which maybe called lateral or vertebral ganglia. (2) 

 A more or less distinct chain, prsevertebral in position, consisting of the 

 semilunar, inferior mesenteric and similar plexuses, which may be 

 called collateral ganglia. (3.) Ganglia situated in the organs and tis- 

 sues themselves, called terminal ganglia. (4.) The ganglia of the pos- 

 terior roots of the spinal nerves. 



The connection between these parts is as follows : the visceral branch 

 -or ramus communicans of each spinal nerve, which is one of the divi- 

 .sions of a typical spinal nerve the others being the dorsal and ventral 

 passes first of all into the lateral chain ; from this chain branches, rami 

 efferentes, pass into the collateral ganglia, and from these again other 

 branches pass off into the organs to end in the terminal ganglia. In the 

 thoracic region the rami communicantes are composed of two parts, 

 white and gray. The former can be traced backwards into both spinal 

 nerve roots of their corresponding spinal nerve ; and in the other direc- 

 tion partly into the lateral sympathetic chain, and partly into the great 

 splanchnic nerves and so into the collateral ganglia without entering the 

 lateral chain at all. The upper wliite rami (from the 2d to 5th), how- 

 ever, proceed upwards and join the superior cervical ganglion, instead of 

 passing downwards into the splanchnics. Other branches go downwards 

 into the lumbar and sacral plexuses. The gray rami of all the spinal 

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