NEUROLOGY 
THE NERVOUS SYSTEM 
The nervous system (Systema nervorum) is a complex mechanism by which the 
organism is brought into functional relation with its environment, and its various 
parts are codrdinated. 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 (Medulla spinalis), and (b) the brain 
(Encephalon). The peripheral nervous system (Systema nervorum periphericum) 
includes—(a) the cranial and spinal nerves with their ganglia, and (b) the sympa- 
thetic nervous system. 
The division into central and peripheral parts is quite arbitrary, and is empioyed 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- 
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 feltwork 
of glia-fibers, many of which are connected with the small gha-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 bundles or tracts (funiculi, fasciculi). The gray matter (Substantia grisea) 
is usually brownish-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-bodies 
and non-medullated processes. 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. They are commonly 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 lymph 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 
1 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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