in the cavities of the cranium and the spinal column. These cavities are
lined with a very tough fibrous membrane, termed the _dura mater_, which
serves as the periosteum of the bones which enter into the formation of
these parts. The surface of the brain and spinal cord is closely
invested with an extremely vascular, areolar tissue, called the _pia
mater_. The numerous blood-vessels which supply these organs traverse
the pia mater for some distance, and, where they pass into the substance
of the brain or spinal cord, the fibrous tissue of this membrane
accompanies them to a greater or less depth. The inner surface of the
dura mater and the outer surface of the pia mater are covered with an
extremely thin, serous membrane, which is termed the _arachnoid_
membrane. Thus, one layer of the arachnoid envelopes the brain and
spinal cord, and the other lines the dura mater. As the layers become
continuous with each other at different points, the arachnoid, like the
pericardium, forms a shut sac, and, like other serous membranes, it
secretes a fluid, known as the _arachnoid fluid_. The space between the
internal and the external layers of the arachnoid membrane of the brain
is much smaller than that enclosed by the corresponding layers of the
arachnoid membrane of the spinal column.

[Illustration: Fig. 56.
Cross-section of spinal cord.]

THE SPINAL CORD is a column of soft, grayish-white substance, extending
from the top of the spinal canal, where it is continuous with the brain,
to about an inch below the small of the back, where it tapers off into a
filament. From this nerve are distributed fibers and filaments to the
muscles and integument of at least nine-tenths of the body.

The spinal cord is divided in front through the middle nearly as far as