Pia matter
The pia mater is a vascular membrane, and derives its blood from the internal carotid and vertebral arteries. It consists of a minute plexus of blood-vessels, held together by an extremely fine areolar tissue. It invests the entire surface of the brain, dipping down between the convolutions and laminae, and is prolonged into the interior, forming the velum interpositum and choroid plexuses of the fourth ventricle. Upon the surface of the hemispheres, where it covers the grey matter of the convolutions, it is very vascular, and gives off from its inner surface a multitude of minute vessels, which extend perpendicularly for some distance into the cerebral substance. At the base of the brain, in the situation of the sub-stantia perforata and locus perforatus, a number of long straight vessels are given off, which pass through the white matter to reach the grey substance in the interior. On the cerebellum, the membrane is more delicate, and the vessels from its inner surface are shorter. Upon the crura cerebri and pons Varolii its characters are altogether changed; it here presents a dense fibrous structure, marked only by slight traces of vascularity.
According to Fohmann and Arnold, this membrane contains numerous lymphatic vessels. Its nerves are derived from the sympathetic, and also from the third, sixth, seventh, eighth, and accessorius. They accompany the branches of the arteries