2. The _pia_, or pia mater, a thin, delicate membrane, containing numerous
blood vessels, that covers the surface of the brain and cord.
3. The _arachnoid_, a membrane of loose texture, that lies between the
dura and the pin.
Finally, within the spaces of the arachnoid is a lymph-like liquid which
completely envelops the brain and the cord, and which, by serving as a
watery cushion, protects them from jars and shocks. Thus the brain and
cord are directly shielded by bones, by membranes, and by the liquid which
surrounds them. They are also protected from jars resulting from the
movements of the body by the general elasticity of the skeleton.
*Summary.*--The nervous system establishes connections between all parts of
the body, and provides a stimulus by means of which they are controlled.
It is made up of a special form of cells, called neurons. The neurons form
the different divisions of the nervous system, and also serve as the
active agents in carrying on its work. Through a side-by-side method of
joining they form the nerves, ganglia, spinal cord, and brain; and by a
method of end-to-end joining they connect places remote from each other,
and provide for nervous movements through the body. The nervous system,
may in some respects be compared to a complicated system of telephony, in
which the chains of neurons correspond to the wires, and the brain and
spinal cord to the central station.
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