CHAPTEE VII. 



THE SYMPATHETIC SYSTEM. 



THE sympathetic system of nerves, when compared with the cerebro- 

 spinal system, presents anatomical peculiarities of arrangement and 

 distribution so distinct and noticeable, that it is naturally regarded as 

 occupying a place by itself. The slender double cord of its main trunk 

 extending throughout the great cavities of the body, the number and 

 scattered position of its ganglia, which are united with each other only 

 by filaments of small size, the frequent and plexiform arrangement of 

 its branches, and the distribution of its terminal fibres to the organs 

 of circulation and nutrition, all form a well marked group of features 

 by which it is easily recognized. But notwithstanding the general im- 

 portance of these characters, the sympathetic nerves and ganglia do not 

 constitute a separate and independent nervous system. Neither the 

 minute structure of their anatomical elements, nor their external con- 

 nections, are essentially different from those of the cerebro-spinal nerves 

 and nervous centres. The sympathetic trunks and branches contain 

 medullated nerve fibres of the same anatomical character as those of 

 the spinal cord and its nerves ; and its ganglia are provided with nerve 

 cells which send off one or more prolongations in the form of nerve 

 fibres. The main peculiarity of intimate structure in the sympathetic 

 nerve fibres is that they are, as a rule, of small diameter, though not 

 smaller than the average of those in the cerebro-spinal nerves. The 

 cells of the sympathetic are also generally of comparatively small size, 

 never, according to Kb'lliker, equalling the largest of those in the gray 

 substance of the spinal cord or the brain ; and they are also charac- 

 terized by the frequency with which they send out only a single pro- 

 longation, thus apparently becoming a source of new fibres. 



But, on the other hand, the cerebro-spinal system contains both 

 fibres and nerve cells of small as well as large size. The posterior roots 

 of all the spinal nerves have connected with them ganglia which are 

 similar in structure to those of the sympathetic system; the fibres 

 which come from the spinal cord simply passing through them, as 

 shown by the observations of Kolliker, and being joined by other fibres 

 originating from the gray matter of the ganglion itself. The same 

 arrangement exists in the ganglia of the cranial nerves, as, for instance, 

 in the Gasserian ganglion of the fifth pair. Thus all the sensitive and 

 mixed cerebro-spinal nerves contain some fibres of ganglionic origin, in 

 addition to those derived directly from the brain or spinal cord. 

 Furthermore, all the sympathetic ganglia receive filaments of communi- 

 (582) 



