The =dura mater= is a dense resistant membrane of white fibrous tissue;
in the fresh state it is bluish-white in color. On account of the
difference in its arrangement within the cranium from that in the spinal
canal it is customary to describe it as consisting of two parts,
cerebral and spinal; these portions are continuous with each other at
the foramen magnum.
The =cerebral dura mater= (Dura mater encephali) is adherent to the
interior of the cranium, and may be regarded as forming an internal
periosteum for the bones here as well as being an envelope of the brain.
Its outer surface is connected with the bony wall of the cranial cavity
by numerous fine fibrous strands and by blood-vessels; hence it appears
rough in many places when separated from the wall. The degree of
adhesion varies greatly at different points. It is most firmly attached
at the various projections, _e. g._, the internal sagittal crest, the
tentorium osseum, the petrosal crest; also at the base and the foramen
magnum. Before the sutures are closed the dura is connected with the
sutural ligaments and through them is continuous with the external
periosteum. The cranial nerves receive sheaths from the dura, which is
thus continuous without the cranium with the epineurium and periosteum.
Along the roof and sides (except as noted above) the adhesion is
relatively slight, and lymph-spaces are said to exist between the dura
and the bone. The internal surface of the dura is smooth and glistening,
since it is lined by endothelium and is moistened by a fluid resembling
lymph; it forms the outer boundary of the subdural space. In accordance
with its double function the dura is composed of two layers, which are,
however, intimately adherent to each other in most places in the adult.
The venous sinuses are channels between the two layers and are lined by
endothelium. They have been described with the other vessels.
Several folds or septa are given off from the inner surface of the dura;
these project into the cavity between the gross subdivisions of the
brain. These septa are: (1) the falx cerebri; (2) the tentorium
cerebelli; and (3) the diaphragma sellæ.
The =falx cerebri= is a sickle-shaped median partition which projects
downward in the great longitudinal fissure between the cerebral
hemispheres. It is attached above to the internal sagittal crest, in
front to the crista galli, and behind to the tentorium osseum. Its
superior border is convex and separates into two layers which inclose
the superior longitudinal sinus. Its lower border is concave and lies
over the corpus callosum. The falx is thick above, but much thinner
below, and in some places cribriform.
In the horse the posterior part of the falx cerebri does not extend to
the corpus callosum, and hence the hemispheres are here in contact and
adherent to each other over a small area. In the ox and pig the falx
does not descend so far as in the horse, while in the dog it
completely separates the hemispheres.
The =tentorium cerebelli= is a crescentic transverse fold which
separates the cerebellum from the posterior poles of the cerebral
hemispheres. It is attached above to the tentorium osseum and laterally
to the petrosal crests. Its lower border is thin, concave, and free; it
forms the upper and lateral boundaries of an opening (Incisura tentorii)
which incloses the mid-brain.
The =diaphragma sellæ= is a small circular fold which roofs over the
sella turcica and the pituitary body. It is perforated centrally by an
opening (Foramen diaphragmatis) for the infundibulum.
The falx cerebelli, a sickle-shaped fold which projects into the
median notch between the cerebellar hemispheres in man, is not present
in the domesticated animals. There is instead merely a slight
thickening of the dura.
The =spinal dura mater= (Dura mater spinalis) forms a tube around the
spinal cord from the foramen magnum to the second or third segment of
the sacrum. It is separated from the periosteum of the spinal canal by a
considerable =epidural space= (Cavum epidurale) which is occupied by
fatty connective tissue and veins. It is held in position chiefly by the
sheaths which it furnishes to the roots of the spinal nerves, and in its
anterior part by two ligaments; the latter connect it with the inferior
atlanto-occipital membrane and with the odontoid process. It is large in
proportion to its contents, but its diameter is not uniform. It is
largest in the atlas, small in the thoracic region, and becomes very
small in its terminal part, where it incloses the delicate filum
terminale of the spinal cord.
The =subdural space= (Cavum subdurale) is the cavity between the inner
surface of the dura mater and the arachnoidea. It is a mere capillary
space which contains just sufficient fluid to moisten its surfaces; this
fluid is usually regarded as lymph, which is replenished by filtration
through the walls of the blood-vessels. The space is in communication
with the lymph-paths of the nerve-sheaths.