Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Nervous System
General Information on the Nervous System
Meninges of the Brain

The Spinal Cord and the Brain are enveloped by three Meninges: the pia mater (or vascular membrane), the arachnoid mater, and the dura mater (Fig. 3.3).

The pia mater is a very thin membrane composed of loose Connective Tissue containing a network of elastic fibers and bundles of Collagen fibers. This membrane is penetrated by numerous Blood Vessels, which it accompanies into the brain tissue. Invaginating into the brain cavities (ventricles) during development, the pia mater forms the choroid plexuses within them. Externally, the membrane is covered by a simple squamous epithelium similar to mesothelium. The pia mater is fused to The surface of the brain by the glial limiting membrane. The membrane is innervated by nerves arising from the Autonomic plexuses that accompany the internal carotid and vertebral Arteries.

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Fig. 3.3. Meninges of the spinal cord:

1 — dura mater, 2 — pia mater, and 3 — arachnoid mater; 4 — dorsal ROOT; 5 — spinal nerve; 6 — spinal ganglion

The arachnoid mater (arachnoidea) is thin and avascular. In the spinal cord region, it is connected to the pia mater by thin connective tissue trabeculae. Both the inner and outer surfaces of the membrane are covered by a continuous layer of flattened Cells. Between the pia and arachnoid mater lies the subarachnoid space, filled with CEREBROSPINAL FLUID. In the spinal cord region, this space reaches 1—2 mm in width, while in the brain its dimensions vary. Over the surface of the gyri, it is practically absent; here, the arachnoid mater fuses with the pia mater, but instead of following it into the sulci, it bridges over them. Thus, a fairly large space is created over the sulci. In certain areas of the brain, these spaces are so large that they are called cisterns (Atl. Fig. 93). The largest of these are located between the Cerebellum and the Medulla Oblongata (cerebellomedullary cistern), in the lateral sulcus of the cerebral hemispheres, between the cerebral peduncles, and between the optic chiasm and the frontal lobes of the hemispheres. All subarachnoid spaces communicate with each other, and through three apertures in the roof of the IV ventricle, they connect with its cavity and the cavities of other ventricles. The outer surface of the arachnoid mater faces the dura mater. In some places, projections (granulations) of the arachnoid mater extend toward the dura mater (see below).

In the caudal region of the spinal cord, the arachnoid mater, along with the pia mater, continues into the filum terminale.

Both membranes, together with the intervening subarachnoid space, constitute a protective and trophic system around the brain. Foreign substances and toxic tissue breakdown products that enter the cerebrospinal fluid are processed by the wall endothelium and macrophages.

The dura mater is the outermost layer, consisting of Cytology/practical/45.html">Dense connective tissue with an Abundance of collagen fibers. Within the vertebral canal, it forms a dense fibrous sac around the spinal cord, which is firmly fused superiorly with the margins of the foramen magnum. Inferiorly, this membrane surrounds the cauda equina and, together with the filum terminale, attaches to the periosteum of the coccyx. The space between the arachnoid and dura mater is called the subdural space. It is filled with fluid that is not cerebrospinal fluid. Between the dura mater and the periosteum of the vertebral canal lies the epidural space, which contains adipose tissue and the venous plexus. Thin connective tissue bands (up to 23 pairs), forming the denticulate ligament, are stretched laterally between the pia and dura mater, anchoring the spinal cord. In the intervertebral and cranial foramina, extensions of the dura mater sheath the roots of the spinal and Cranial Nerves, fuse with the margins of the foramina, and continue into the nerve sheaths.

There is no subdural space in the cranium. The dura mater is firmly fused with the internal periosteum of the BONES OF THE cranial base. In the region of the cranial vault, the dura mater forms septa that extend between PARTS OF THE brain, preventing them from pressing against each other (Atl. Fig. 94). The largest of these is the falx cerebri, which projects in the sagittal plane between the cerebral hemispheres. It begins anteriorly at the crista galli of the Ethmoid bone and extends posteriorly to the superior surface of the tentorium cerebelli. The falx cerebelli lies between the cerebellar hemispheres. The occipital lobes of the cerebral hemispheres are separated from the cerebellum by the tentorium cerebelli, which is stretched between the superior borders of the petrous parts of the temporal bones over the posterior cranial fossa. In the anterior part of the tentorium cerebelli, There is a notch through which the Brainstem passes. The diaphragma sellae attaches to the margins of the sella turcica, enclosing the cavity that houses the Pituitary Gland.

At the sites where the septa originate, the dura mater splits to form endothelium-lined channels—the Dural Venous Sinuses, which leave impressions in the form of wide grooves on the inner surface of the Skull (Atl. Fig. 20). Cerebral Veins drain into these sinuses. Their non-collapsible walls keep the lumina of the sinuses constant, which facilitates the free drainage of blood and prevents its congestion in The Skull and brain. Arachnoid granulations project into the venous sinuses of the dura mater (Atl. Fig. 95). These granulations ensure the outflow of cerebrospinal fluid from the subarachnoid spaces into the venous sinuses, thereby maintaining constant intracranial pressure. Granulations appear in the third year of life and increase in size with age.

Along the superior margin of the falx cerebri runs the superior sagittal sinus. On either side of it, between the layers of the dura mater, lie numerous lateral lacunae into which the arachnoid granulations project. The posterior end of the sinus drains into the largest of all—the transverse sinus, which lies at the Base of the tentorium cerebelli in the corresponding groove of the Occipital bone. The transverse sinus continues as the sigmoid sinus, which descends to the jugular foramen and continues into the superior bulb of the Internal jugular vein (Atl. Fig. 94). The inferior sagittal sinus runs along the inferior margin of the falx cerebri and drains into the straight sinus.