The nervous system is a complex mechanism by which the organism is
brought into functional relation with its environment, and its various
parts are coördinated. 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 (_b_) the =brain=
(Encephalon).
The =peripheral nervous system= (Systema nervorum periphericum) includes
(_a_) the =cerebral= and =spinal nerves= with their =ganglia=, and (_b_)
the =sympathetic 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
outgrowths 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 inclosed
in a protecting and insulating sheath, and united into bundles by
connective 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 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.[186]
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 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 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 anatomical descriptions.
=Nerves= 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 connections as =cranial= or
=cerebral=, =spinal=, and =sympathetic=. The groups 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 peripheral or central direction. On the same basis the nerve-fibers
are designated =efferent= and =afferent= respectively. A bundle of
fibers which passes from one nerve-trunk to another is called an
=anastomotic branch= (Ramus anastomoticus). In some situations the
exchange of branches between adjacent nerves is so free as to constitute
a =nerve-plexus= (Plexus nervorum). The term =ramus communicans= is
properly restricted to branches which connect the ventral divisions of
the spinal nerves with adjacent ganglia or nerves of the sympathetic
system. The terminal twigs of the nerves are designated according to
their distribution as =muscular branches= (Rami musculares), =cutaneous
nerves= (Nervi cutanei), and =articular nerves= (Nervi articulares). The
muscular branches are motor in function, the cutaneous and articular
sensory, but all contain vasomotor fibers which control the caliber of
the blood-vessels.