CHAPTEE III. 



GENERAL ARRANGEMENT OF THE VARIOUS 

 PARTS OF THE NERVOUS SYSTEM. 



IN man and the vertebrate animals the nervous system may be divided 

 into two secondary systems, or groups of nervous centres with their 

 commissural fibres and nerves. These are, first, the ganglionic or sym- 

 pathetic, and secondly, the cerebro-spinal system. 



Ganglionic System. The ganglionic or "sympathetic" system oc- 

 cupies mainly the great cavities of the body. It is connected by its 

 nervous branches and ramifications with the internal organs concerned 

 in the functions of nutrition, and more especially with the heart and 

 bloodvessels, which it follows throughout their peripheral distribution. 

 Its especial anatomical character consists in its being composed of numer- 

 ous separate masses or collections of gray matter, of small size and 

 rounded form, called "ganglia;" from which circumstance the whole 

 ganglionic system takes its name. These ganglia, connected with each 

 other by slender nervous filaments, form a double chain of distinct but 

 associated nervous centres, situated in front of the spinal column 

 throughout the neck and thorax ; while in the abdomen they are at 

 certain points fused together into larger and more irregularly shaped 

 masses upon the median line. There are also scattered ganglia about 

 the head, outside the cranial cavity ; and everywhere the ganglia or 

 their nerves receive some fibres of communication from the other divi- 

 sion of the nervous system. The scattered arrangement of the sym- 

 pathetic ganglia, and their deep situation among the thoracic and 

 abdominal organs, have hitherto prevented a complete investigation of 

 their functions ; and it is doubtful how far any one portion exercises a 

 central or controlling influence over the remainder. 



Cerebro- Spinal System. The cerebro-spinal system, as its name indi- 

 cates, is made up of the brain and spinal cord as the great nervous cen- 

 tres, with the nerves which originate from them and which are distributed 

 to the voluntary muscles and integument, the organs of special sense, 

 and the commencement and termination of the internal passages of 

 the body. It is especially distinguished by the fact that its deposits of 

 gray substance, instead of being distributed in separate nodules as in 

 the ganglionic system, are collected into two principal continuous masses, 

 the brain and the spinal cord, occupying the cranial and spinal cavities, 

 where they are enveloped and connected by tracts of white substance, 

 which often conceal in an external view the divisions between them. 



The cerebro-spinal nervous system is also distinguished by a nearly 

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