CHAPTER IX. 



NEUROLOGY— THE STUDY OP THE NERVOUS SYSTEM. 



GENEEAIi CONSIDEEATIONS — THE MTELON (SPINAL COKD) AND ITS NERVES — THE BRA- 

 CHIAL PLEXUS — THE VAGtrS NERVE — THE STMPATHIC SYSTEM — THE RELATIONS OP 

 THE STMPATHIC AND MYELENCBPHALIC (CBRBBRO-SPINAL) SYSTEMS — STRUCTURE 

 OP NERVOUS MATTER. 



§ 993. General Considerations. — Nerves. — Throughout the 

 "body, distributed to all organs and membranes, there are white cords 

 which are neither hollow like the vessels nor inextensible like the 

 tendons, but composed of a greater or less number of fibers of a 

 peculiar structure (§ 1048). These cords are called nerves ; the 

 larger ones are also distinguished as trunks, the smaller as 'branches, 

 the yet smaller as t/udgSy and the final subdivisions as fibers or ter- 

 fninal filaments. 



§ 994. Ganglia. — The peripheral ends of the nerves are distributed to the various tis- 

 sues constituting the muscles, bones, viscera, membranes, etc. Their central ends, how- 

 ever, are sooner or later traceable to collections of cells (§ 1048, B), with which they are 

 more or less closely and directly connected. Such collections of cells, whether or not 

 intermingled with fibers, are called ganglia, or said to constitute ganglionic tissue. 



§ 995. Alba and Cinerea. — In the ganglia the gray protoplasm of the nerve ceUa 

 imparts to the mass a more or less decided gray color. Hence the ganglionic tissue is 

 commonly spoken of as the gray matter, or more technically the (substantia) cinerea. 



But although the central (ental) part of each nerve fiber is a band of gray protoplasm, 

 it is in most cases so completely covered by a white substance (myeline, medullary sheath 

 or white substance of Schwann), that the prevailing color of the fibrous nervous tissue is 

 white, and it is commonly known as the white substance, or more technically the (substan- 

 tia) alba. 



§ 996. Primary Divisions. — The nerves and ganglia may be 

 conveniently considered as forming two great divisions which are 

 tolerably distinct in location and functions, but are nevertheless 

 anatomically connected and physiologically associated : they are 

 the sympafhic (sympathetic) and the myelencepjialic (cerebro-spinal) 

 nervous systems. 

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