Sheetlike extensions of the inner surface of the dura mater form partitions between parts of the brain, supporting those parts within the cranial cavity and carrying the arachnoid to some of the deeper parts of the brain. In the spinal canal, the vertebrae have their own periosteum, and the dura mater forms a separate tube surrounding the spinal cord (see Fig. 12.29).
The arachnoid is a delicate sheet of connective tissue adjacent to the inner surface of the dura.
The arachnoid abuts on the inner surface of the dura and extends delicate arachnoid trabeculae to the pia mater on the surface of the brain and spinal cord. The weblike trabeculae of the arachnoid give this tissue its name (Gr., resembling a spider’s web). Trabeculae are composed of loose connective tissue fibers containing elongated fibroblasts. The space bridged by these trabeculae is the subarachnoid space; it contains the cerebrospinal fuid (Fig. 12.31).
The pia matter lies directly on the surface of brain and spinal cord.
The pia mater (L., tender mother) is also a delicate connective tissue layer. It lies directly on the surface of the brain and spinal cord and is continuous with the perivascular connective tissue sheath of the blood vessels of the brain and spinal cord. Both surfaces of the arachnoid, the inner
FIGURE 12.31 • Schematic diagram of the cerebral meninges.
The outer layer, the dura mater, is joined to adjacent bone of the cranial cavity (not shown). The inner layer, the pia mater, adheres to the brain surface and follows all its contours. Note that the pia mater follows the branches of the cerebral arteries as they enter the cerebral cortex. The intervening layer, the arachnoid, is adjacent but not attached to the dura mater. The arachnoid sends numerous, weblike arachnoid trabeculae to the pia mater. Located between the arachnoid and the pia mater is the subarachnoid space; it contains cerebrospinal fluid. The space also contains the larger blood vessels (cerebral arteries) that send branches into the substance of the brain.
surface of the pia mater, and the trabeculae are covered with a thin squamous epithelial layer. Both the arachnoid and pia mater fuse around the opening for the cranial and spinal nerves as they exit the dura mater.
The blood–brain barrier protects the CNS from fluctuating levels of electrolytes, hormones, and tissue metabolites circulating in the blood vessels.
The observation more than 100 years ago that vital dyes injected into the bloodstream can penetrate and stain nearly all organs except the brain provided the first description of the blood–brain barrier. More recently, advances in microscopy and molecular biology techniques have revealed the precise location of this unique barrier and the role of endothelial cells in transporting essential substances to the brain tissue.