erous
membrane, the outer, fibrous layer constituting the endosteum of the
skull, the inner, serous layer forming one of the coverings of the
brain. Between the fibrous layer and the bone the meningeal vessels
ramify; and along certain lines the two layers split to form channels
in which run the cranial venous sinuses. Inside the dura, and
separated from it by a narrow space--the _sub-dural space_--lies the
_arachno-pial membrane_, consisting of an outer (_arachnoid_) layer
which envelops the brain but does not pass into the sulci, and a
highly vascular inner layer--the _pia mater_--which closely invests
the brain and lines its entire surface.

The space between these layers--the _sub-arachnoid space_--is
traversed by a network of fine fibrous strands, in the meshes of which
the cerebro-spinal fluid circulates. Each nerve-trunk as it leaves the
skull or spinal canal carries with it a prolongation of each of these
membranes and their intervening spaces. The membranes gradually become
lost in the fibrous sheaths of the nerves, and the sub-dural and
sub-arachnoid spaces become continuous with the lymph spaces of the
nerves.

The _cerebro-spinal fluid_ is secreted by the choroid plexuses and
fills the cerebral ventricles, the central canal of the cord, the
sub-dural and sub-arachnoid spaces, and the sheaths of the
intra-cerebral blood vessels. At the base of the brain, particularly
in the posterior fossa, the sub-arachnoid space is wider than
elsewhere, forming "cisterns" filled with cerebro-spinal fluid which
supports the cerebral structures. Through the foramen of Magendie in
the roof of the fourth ventricle the sub-arachnoid fluid of the
cranial cavity communicates with that of the vertebral canal.

Although it differs in its chemical constitution from true lymph, the
cerebro-spinal fluid seems to functionate as lymph, in addition to
acting as a lubricating agent, and playing a part in regulating the
vascular supply of the brain. In cases of cerebral haemorrhage,
abscess, tumour, or depressed fracture, room is made up to a certain
point for the extraneous matter by displacement of cerebro-spinal
fluid.

_Vascular supply._--The free anastomosis between the vessels entering
into the formation of the circulus arteriosus (circle of Willis)
ensures an abundant supply of blood to the brain. The larger arteries
run in the sub-arachnoid space and give off branches which ramify in
the pia mater before entering the cerebral