<P> The section of the pia mater enveloping the brain is known as the cranial pia mater . It is anchored to the brain by the processes of astrocytes, which are glial cells responsible for many functions, including maintenance of the extracellular space . The cranial pia mater joins with the ependyma, which lines the cerebral ventricles to form choroid plexuses that produce cerebrospinal fluid . Together with the other meningeal layers, the function of the pia mater is to protect the central nervous system by containing the cerebrospinal fluid, which cushions the brain and spine . </P> <P> The cranial pia mater covers the surface of the brain . This layer goes in between the cerebral gyri and cerebellar laminae, folding inward to create the tela chorioidea of the third ventricle and the choroid plexuses of the lateral and third ventricles . At the level of the cerebellum, the pia mater membrane is more fragile due to the length of blood vessels as well as decreased connection to the cerebral cortex . </P> <P> The spinal pia mater closely follows and encloses the curves of the spinal cord, and is attached to it through a connection to the anterior fissure . The pia mater attaches to the dura mater through 21 pairs of denticulate ligaments that pass through the arachnoid mater and dura mater of the spinal cord . These denticular ligaments help to anchor the spinal cord and prevent side to side movement, providing stability . The membrane in this area is much thicker than the cranial pia mater, due to the two - layer composition of the pia membrane . The outer layer, which is made up of mostly connective tissue, is responsible for this thickness . Between the two layers are spaces which exchange information with the subarachnoid cavity as well as blood vessels . At the point where the pia mater reaches the conus medullaris or medullary cone at the end of the spinal cord, the membrane extends as a thin filament called the filum terminale or terminal filum, contained within the lumbar cistern . This filament eventually blends with the dura mater and extends as far as the coccyx, or tailbone . It then fuses with the periosteum, a membrane found at the surface of all bones, and forms the coccygeal ligament . There it is called the central ligament and assists with movements of the trunk of the body . </P> <P> In conjunction with the other meningeal membranes, pia mater functions to cover and protect the central nervous system (CNS), to protect the blood vessels and enclose the venous sinuses near the CNS, to contain the cerebrospinal fluid (CSF) and to form partitions with the skull . The CSF, pia mater, and other layers of the meninges work together as a protection device for the brain, with the CSF often referred to as the fourth layer of the meninges . </P>

What connects the pia mater to the dura mater of the spinal cord