Pia Mater

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.