THE SYMPATHETIC SYSTEM 



1029 



convey inpulses toward the central system (sensory sympathetic neurones). 

 While it is very probable that certain of the simpler reflexes of the splanchnic 

 organs may be mediated by the sympathetic system alone, 3'et the sympathetic 

 is by no means independent of the cranio-spinal system, but is rather, both ana- 

 tomically and functionally merely a part of one continuous whole. Throughout, 

 it shares its domain of termination with cranio-spinal fibres, chiefly of the sensory 

 variety, and most of its rami and terminal branches carry a few cranio-spinal 

 fibres toward their areas of distribution. Likewise the cranio-spina,l nerves carry 

 numerous sympathetic fibres gained by way of rami connecting the two systems. 



Like the cranio-spinal system, the sympathetic consists of cell-bodies, each of which gives 

 off one axone. In addition, the cell-bodies give off numerous dichotomously branched den- 

 drites by which their receptive surfaces are increased, and they are accumulated into ganglia, 

 large and small. The larger gangha have more or less constant positions, shapes, and arrange- 

 ments, while the smaller, some of which are microscopic, are scattered throughout the body in 

 a seemingly more indefinite manner. The axones or fibres arising in these gangUa are given off 

 in trunks and rami which associate the gangUa with each other or with the cranio-spinal system, 

 or which pass from the gangha to be distributed directly upon their allotted elements. 



The sympathetic fibres arising from the ganglia are, for the most part, either totally non- 

 meduUated or partially meduUated. Some fibres are completely medullated near their cells 

 of origin, but lose their medullary sheaths before reaching their terminations. Some of them 

 possess complete medullary sheaths throughout, but in no cases are the sheaths as thick or 

 well developed as is the rule with the cranio-spinal fibres. Thus, nerve-trunks and rami in 

 which sympathetic fibres predominate appear greyish in colour and more indefinite, as dis- 

 tinguished from those of the cranio-spinal nerves, which always appear a ghstening white, due 

 to light beiBg reflected from the emulsified myelin of the sheaths of their fibres. 



Origin of the sympathetic system.— Not only must the cranio-spinal and sympathetic 

 systems be considered anatomically continuous and dependent, but also the neurones of the 

 two systems have a common origin, namely, the ectoderm of the dorsal mid-line of the embryo. 

 The cells of the ganglion crest (see p. 754) become arranged in segmental groups and soon 

 separate into two varieties: — those which will remain near the spinal cord and develop into 

 the spinal ganglia, and those which, during the growth processes, migrate and become displaced 

 further into the periphery and form the sympathetic gangha. 



Fig. 785. — Scheme showing the Connection between the Sympathetic and the Chanio- 

 spiNAL and Central Nervous Systems. 



Spinal ganglion neurone 

 to capsule of ganglion 



Gray ramus communicans 

 White ramus communicans 



Sympathetic ganglion 1 Gangliated 

 - Sympathetic trunk ) trunk 

 ' Sympathetic cell body in spinal 

 ganglion 



Posterior primary division 1 Spinal 

 Anterior primary division J nerve 



■v Afferent spinal fibre 

 ^ Efferent visceral fibre 



^^^ ''Gray ramus communicans 



^^ ^White ramus communicans 



Sensory sympathetic neurone 

 Branch to prevertebral ganglion 



In the development of the sympathetic system the migration from the 

 vicinity of the central system occurs to varying extents, so that in the adult the 

 cells comprise three general groups of ganglia situated different distances away 

 from the central nerve axis. — (1) A large portion of the cells remain near the 

 central system and form a linear series' of ganglia which, with the trunks con- 

 necting them, become two gangliated nerve trunks extending along each side, 

 proximal to and parallel with the vertebral column; (2) a still larger portion of the 

 cells migrate further toward the periphery and are accumulated into ganglia 

 which assume an intermediate position and which, with the rami associating them 

 with each other and with other structures, form a series of great prevertebral 



