CHAPTEE IX. 



NEUROLO&T— THE STUDY OF THE NERVOUS SYSTEM. 



QEXEEAL CONSIDERATIONS — THE MTELON (SPINAL COKD) AND ITS NERVES— THE BRA- 

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

 THE 8YMPATHIC AND MYELEKCBPHALIC (CBRBBRO-SPINAL) SYSTEMS — STRUCTURE 

 OF NERVOUS MATTER. 



§ 993. G-eneral 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 nerms ; the 

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

 the yet smaller as twigs, and the final subdivisions Si's fibers or ter- 

 minal filaments. 



% 994: Oanglia. — 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 Oinerea.—lu the ganglia the gray protoplasm of the nerve cells 

 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, meduUa/ry sheath 

 or white mbstanae 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 sympatMc (sympathetic) ^M^VoQmyel&ncejphalic (cerebro-spinal) 

 nervous systems. 

 24 



