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.
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