The =pia mater= is a delicate and very vascular membrane which invests
closely the surface of the brain and spinal cord and sends processes
into their substance. It also furnishes sheaths to the nerves, which
blend outside of the dural sac with the epineurium.
The =pia mater of the brain= (Pia mater encephali) follows accurately
all the inequalities of the surface, dipping into all the fissures and
sulci of the cerebrum, and into the larger fissures of the cerebellum.
Its external surface, which forms the inner boundary of the subarachnoid
space, is covered by a layer of endothelium. From its deep face numerous
trabeculæ are given off which penetrate into the substance of the brain
and cord, forming a path for the blood-vessels, and concurring with the
neuroglia in forming the supporting tissue of the nervous substance
proper. The larger blood-vessels of the brain lie within the
subarachnoid space, but the smaller vessels ramify in the pia, forming
rich plexuses. The twigs which penetrate into the gray matter are
inclosed in pial sheaths. They are end-arteries, _i. e._, constitute the
entire supply of the district which they enter and do not anastomose
with adjacent vessels.
Two important folds of the pia extend into the two great transverse
fissures of the brain; these processes overlie the third and fourth
ventricles and are known as the =telæ chorioideæ= of these cavities.
They constitute paths for the deeper vessels and their edges contain
vascular convolutions which are known as chorioid plexuses. They will
receive further consideration more appropriately later.
The =spinal pia mater= (Pia mater spinalis) is thicker and denser than
that of the brain. It has a strong outer layer of fibrous tissue, most
of the fibers of which are longitudinal. The inner layer is vascular and
adheres closely to the surface of the cord because it sends numerous
processes into it. It sends a fold into the median ventral fissure and
also helps to form the median dorsal septum of the cord. Along the
median ventral line it forms a band-like thickening, the linea
splendens, along which the ventral spinal artery runs. On each side the
pia mater gives off a strong longitudinal band, the =ligamentum
denticulatum=, which is connected externally with the dura mater. The
inner or pial border extends in a line between the dorsal and ventral
roots of the nerves. The outer or dural border is denticulated and to a
large extent free. The denticulations are attached to the dura between
the nerve-roots.