THE NERVOUS SYSTEM 



THE SYMPATHETIC SYSTEM 



The so-called sympathetic system is that portion of the nervous system which is 

 especially concerned in the distribution of impulses to the glandular tissues, to the 

 muscle of the heart, and to the non-striated muscular tissue of the body wherever 

 found. Since these tissues are most abundant in and largely comprise the viscera or 

 splanchnic organs of the body, the largest and most evident of the structures com- 

 prising the sympathetic system are found either in or near the cavities containing the 

 viscera. However, the finer divisions of the system ramify throughout the whole 

 body, supplying vaso-motor fibres to the blood-vessels throughout their course, con- 

 trolling the glands of the skin, and supplying pilo-motor fibres for the hairs, form- 

 ing intrinsic plexuses within the walls of the viscera, and certain of its fibres convey 

 impulses towards the central system (sensory sympathetic neurones). While it is 

 very probable that certain of the simpler reactions of the splanchnic organs may be 

 mediated by the sympathetic system alone, yet the sympathetic is by no means in- 

 dependent of the cerebro-spinal system, but is rather, both anatomically and func- 

 tionally, merely a part of one continuous whole. Throughout, it shares its domain 

 of termination with cerebro-spinal fibres, chiefly of the sensory variety, and most of 

 its rami and terminal branches carry a few cerebro-spinal fibres towards their areas 

 of distribution. Likewise the cerebro-spinal nerves carry numerous sympathetic 

 fibres gained by way of rami connecting the two systems. 



FIG. 715. DIAGRAM SHOWING (1) THE GROWTH OF THE PRIMITIVE GANGLIA FROM THE NEURAL 

 CREST; (2) THE DIVISION OF THE PRIMITIVE GANGLIA INTO SPINAL AND SYMPATHETIC 

 PORTIONS, AND (3) THE FORMATION OF THE NERVES. 



NEURAL CREST 

 (Disappearing) 



SUB f ACE ECTODERM 



ECTODERM 

 NEURAL CREST 



KEURAL TUBE 



NEURAL TI'BE 



SYMPATHETIC OAltGLIOX 



VEXTRA T. 

 SERVE-ROOT 



SCRFA CE 

 ECTODERM 

 SPINAL CORD 



DORSAL 

 KERVE-ROOT 



SYMPATHETIC OAlfGLIOX 



Like the cerebro-spinal system, the sympathetic consists of cell-bodies, each of 

 which gives off one axone. In addition, the cell-bodies give off numerous dichoto- 

 mously branched dendrites by which their receptive surfaces are increased, and they 

 are accumulated into ganglia, large and small. The larger ganglia have more or less 

 constant positions, shapes, and arrangements, 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 ganglia are given off in trunks and rami which 

 connect the ganglia with each other or with the cerebro-spinal system, or which pass 

 from the ganglia to be distributed directly upon their allotted elements. 



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

 totally non-inedullated or partially medullated. Some fibres are 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 cerebro-spinal fibres. Thus, 

 nerve-trunks and rami in which sympathetic fibres predominate appear greyish in 

 colour and more indefinite, as distinguished from those of the cerebro-spinal nerves, 

 which always appear a glistening white, due to light being reflected from the 

 myelin of the sheaths of their fibres. 



Origin of the sympathetic system. Not only must the cerebro-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 neural crest (see p. 753) become 

 arranged in segmental groups and soon separate into two varieties: those which will 



