The pia mater is a delicate vascular membrane which closely invests the nervous substance.
It follows accurately the contour of the surface of the brain, dipping into all the fissures of the cerebrum and cerebellum - in the smaller sulci of the latter, however, a double layer cannot be distinctly made out. Processes or folds of this membrane project into some of the ventricles of the brain, and are separated from the ventricular cavities only by a layer of epithelium. These folds form the velum interpositum and choroid plexuses, which will be described with the anatomy of the ventricles. The blood-vessels, which divide freely in the subarachnoid tissue, subdivide into the pia mater, forming by their inosculations fine networks from which innumerable minute vessels proceed to penetrate the nervous substance.