free on its inner surface. The
inner surface has the appearance of a serous membrane, and when
examined microscopically is seen to consist of a layer of squamous
endothelial cells. Hence the dura mater is sometimes called a
fibro-serous membrane. The dura mater is well provided with lymph
vessels, which in all probability open by stomata on the free inner
surface. Between the dura mater and the subjacent arachnoid membrane
is a fine space containing a minute quantity of limpid serum, which
moistens the smooth inner surface of the dura and the corresponding
smooth outer surface of the arachnoid. It is regarded as equivalent to
the cavity of a serous membrane, and is named the _sub-dural space_.

_Arachnoid Mater._--The arachnoid is a membrane of great delicacy and
transparency, which loosely envelops both the brain and spinal cord.
It is separated from these organs by the pia mater; but between it and
the latter membrane is a distinct space, called _sub-arachnoid_. The
sub-arachnoid space is more distinctly marked beneath the spinal than
beneath the cerebral parts of the membrane, which forms a looser
investment for the cord than for the brain. At the base of the brain,
and opposite the fissures between the convolutions of the cerebrum,
the interval between the arachnoid and the pia mater can, however,
always be seen, for the arachnoid does not, like the pia mater, clothe
the sides of the fissures, but passes directly across between the
summits of adjacent convolutions. The sub-arachnoid space is
subdivided into numerous freely-communicating loculi by bundles of
delicate areolar tissue, which bundles are invested, as Key and
Retzius have shown, by a layer of squamous endothelium. The space
contains a limpid cerebro-spinal fluid, which varies in quantity from
2 drachms to 2 oz., and is most plentiful in the dilatations at the
base of the brain known as _cisternae_. It should be clearly
understood that there is no communication between the subdural and
sub-arachnoid spaces, but that the latter communicates with the
ventricles through openings in the roof of the fourth, and in the
descending cornua of the lateral ventricles.

When the skull cap is removed, clusters of granular bodies are usually
to be seen imbedded in the dura mater on each side of the superior
longitudinal sinus; these are named the _Pacchionian bodies_. When
traced through the dura