e pelvis (Figs. 125 and 133). The number of
ganglia in each of these chains is about twenty-four. They are connected
on either side by the right and left sympathetic nerves which extend
vertically from ganglion to ganglion. In addition to the ganglia forming
these chains, important ones are found in the head (outside of the cranial
cavity) and in the plexuses of the thorax and the abdomen.

The sympathetic ganglia receive nerves from the central division of the
nervous system, but connect with glands, blood vessels, and the intestinal
walls through fibers from their own cell-bodies. Some of these latter
fibers join the spinal nerves, and some blend with each other to form
small sympathetic nerves.

*Protection of Brain and Spinal Cord.*--On account of their delicate
structure, the brain and spinal cord require the most complete protection.
In the first place, they are surrounded by the bones of the head and
spinal column; these not only shield them from the direct effects of
physical force, but by their peculiar construction prevent, to a large
degree, the passage of jars and shocks to the parts within. In the second
place, they are surrounded by three separate membranes, as follows:

1. The _dura_, or dura mater, a thick, dense, and tough membrane which
lines the bony cavities and forms supporting partitions.

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