NEUROLOGY 



THE NERVOUS SYSTEM 



The nervous system (Sj^stema nervorum) 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 ner- 

 vorum centrale) comprises — (a) the spinal cord (IMeduUa spinalis), and (&) the brain 

 (Encephalon) . The peripheral nervous system (Systema nervorum periphericum) 

 includes — (a) the cranial and spinal nerves with their ganglia, and (&) the sympa- 

 thetic 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 witliin the central system, and therefore constitute an integral part of the latter. 

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

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

 growths from the cell-body and conduct impulses to or from the cell; they vary greatly in length, 

 some being 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 enclosed in a protecting 

 and insulating sheath, and united into bundles by areolar tissue. The neurones are embedded 

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

 of glia-fibers, many of which 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 ingro'irths 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 meduUated 

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

 small bundles or tracts (funiculi, fasciculi). The gray matter (Substantia grisea) 

 is usually browTiish-gray in color, often with a tinge of pink. It is softer than the 

 white substance and much more vascular. It is composed chiefly of cell-l)odies 

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

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

 jelly-like. 



Ganglia are gray masses found on the dorsal roots of the spinal nerves and 

 on the course of many nerves. Thej- are commonl}^ ovoid in form, but some 

 are irregular in shape and branched. They vary greatly in size; some are micro- 

 scopic, while others are several inches long. They are enclosed 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 peripherally and 

 centrally. In origin and function the spinal ganglia belong properly to the central 

 system, but it is customary to include them with the peripheral part in gross ana- 

 tomical descriptions. 



Nerves (Nervi) are conducting trunks composed of bundles of parallel nerve- 

 fibers. They are enveloped in a fibro-elastic sheath, the epineurium, which contains 

 the blood and Ijinph vessels. They are classified according to their central connec- 

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

 the nerve-fibers are processes are termed the nuclei of origin (Nuclei originis) or 

 terminal nuclei (Nuclei terminales) of the nerves, according as the latter conduct 



' Limitations of space and the purpose of this work preclude consideration of the finer 

 structure of the nervous system, for which ample literature is available. 



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