NEUROLOGY 



THE NERVOUS SYSTEM 



The nervous system is a complex mechanism by which the organism is brought 

 into functional relation with its environment, and its various parts are coordinated. 

 For purposes of gross description it is divided primarily into two parts, central 

 and peripheral. 



The central nervous system (Systema nervorum centrale) comprises (a) the 

 spinal cord (Medulla spinalis), and (6) the brain (Encephalon). 



The peripheral nervous system (Sj^stema nervorum periphericum) includes 

 (a) the cerebral and spinal nerves with their ganglia, and (h) the sjrmpathetic 

 nervous system. 



The division into central and peripheral parts is quite arbitrary and is employed purely 

 as a matter of convenience of description. The fibers of which the nerves are composed either 

 arise or end within the central system, and therefore constitute an integral part of the latter. 

 The structural and functional unit of the nervous system is the neurone, which consists of the 

 cell-body, usually termed the nerve-cell, and all its processes. The processes arise as out- 

 growtlis from the cell-body and conduct impulses to or from the cell. They vary greatly in 

 length, some Ijeing less than a millimeter long, while others extend from a cell in the spinal cord 

 to the distal end of a limb. A nerve is composed of such processes, usually inclosed in a protecting 

 and insulating sheath, and uniteil into bundles by connective tissue. The neurones are cmlicdded 

 in a peculiar supporting tissue termed neuroglia. This consists of a very intricate feltwork 

 of glia-fibers, many of whicii are connected with the small glia-cells or astrocytes. In addition 

 the nervous tissue is invested closely by a vascular layer of connective tissue, the pia mater, from 

 which ingrowths extend into the nervous substance proper.' 



To the naked eye the central nervous system appears to be composed chiefly 

 of two kinds of substance, white and gray. The white matter (Substantia alba) is 

 dead white in color and is soft in the natural state. It consists largely of medullated 

 nerve-fibers, packed closely together and arranged more or less clearly in large or 

 small l)undles or tracts (Funiculi, fasciculi). The gray matter (Substantia grisea) 

 is usually l)rownish-gray in color, often with a tinge of j)ink. It is softer than the 

 white substance and nuich more vascular. It is composed chiefly of cell-bodies 

 and non-medullated processes. In some situations it is modified to form the 

 gelatinous substance (Substantia gelatinosa), which is pale vellowish-gray and 

 jelly-like. 



Ganglia are gray masses found on the dorsal roots of the s])inal nerves and on 

 the course of many nerves. They are commonly ovoid in form and are inclosed in 

 a fibrous capsule. They are composed largely of the cell-bodies of neurones, but 

 have connected with and passing through them nerve-fibers (processes) which 

 extend jieripherally and centrally. In origin and function the spinal ganglia 

 belong properly to the central system, but it is customar,y to include them with the 

 peripheral part in gross anatomical descriptions. 



Nerves are conducting trunks comiiosed of bimdles of parallel nerve-fibers. 

 They are enveloped in a fibro-elastic sheath, the e])ineiu'iiun, which contains the 

 blood and lynijih vessels. They ar(> classified according to their central connc'c- 

 tions as cranial or cerebral, spinal, and sympathetic. The grotips of cells of which 

 the nerve-fibers are processes are termed the nuclei of origin or termination of 

 the nerves, according as the latter conduct impulses in a])eri])lH'ial or central direc- 

 tion. On the same basis the nerve-fillers are designated efferent and afferent 



' Limitations of space and tlic purpose of tliis work preclude consideration of the finer 

 structure of the nervous system, for wliicii aniiile literature is available. 



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