Human Anatomy: Part 2 - K. A. Dyubenko, A. K. Kolomiysev, Yu. B. Chaykovsky 2008

Specialized Part
Nervous System (systema nervosum) — Specialized Part
Meninges of the Brain

The Meninges of the Brain (Fig. 169) are Connective Tissue membranes that cover the brain and form a nearly closed cavity filled with CEREBROSPINAL FLUID, which protects it from injury. The spinal meninges are continuous with those of the brain. The cranial meninges include: the dura mater (dura mater cranialis), the arachnoid mater (arachnoidea mater cranialis), and the pia mater (pia mater cranialis).

The dura mater of the brain, dura mater cranialis, is the outermost of the meninges (Fig. 169). It is a dense, laminar Structure, 0.7-1 mm thick, closely applied to the internal surface of the cranial bones. It is formed by thick bundles of Collagen fibers and thin bundles of elastic fibers that lie closely together and are oriented in various directions, providing the membrane with significant strength and elasticity. The dura mater serves as the internal periosteum for the BONES OF THE Skull. In areas of apertures, sutures, protrusions, and vascular grooves, it fuses tightly with the cranial bones*. The internal surface of the dura mater is smooth with a bluish sheen, and it is loosely connected to the arachnoid mater. Between the dura and arachnoid maters lies a multi-layered cellular structure (2-8 layers) known as the subdural neuroepithelium. The subdural space, spatium subdurale, forms only under pathological conditions (subdural hematomas) (K. Andres, 1967). In certain areas, the dura mater splits into two layers, between which venous sinuses and the cavity for the trigeminal ganglion, cavitas trigeminalis, are formed. A number of processes extend from the dura mater: the falx cerebri, tentorium cerebelli, falx cerebelli, and Diaphragm of the sella turcica, which project between the structures of the brain.

* Under pathological conditions, it may detach, creating a cleft between it and the inner surface of the bones—the so-called epidural space; if the bones are damaged, hematomas may form here.

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Fig. 169. Diagram of the cerebral meninges and arachnoid granulations (after N. V. Kolesnikov)

1 - dura mater; 2 - subdural space; 3 - arachnoid mater; 4 - pia mater; 5 - subarachnoid space; 6 - Blood vessel; 7 - bone; 8 - arachnoid granulations

The falx cerebri (Fig. 169 B) extends from the dura mater of the vault and projects into the longitudinal fissure between the cerebral hemispheres. In shape, it resembles a sickle-like process with a thickness of 0.7-0.9 mm. Anteriorly, the falx attaches to the crista galli of the Ethmoid bone; posteriorly, to the cruciate eminence of the Occipital bone, where it fuses with the tentorium cerebelli. The lower free edge of the process reaches the corpus callosum. The shape and dimensions of the falx cerebri depend on the constitutional type of the individual.

The tentorium cerebelli is horseshoe-shaped, occupies a horizontal position in the cranial cavity, and separates the Cerebellum from the occipital lobes. Laterally, it attaches to the superior borders of the petrous part of the Temporal bone; posteriorly, to the cruciate eminence.

Along the superior midline, the tentorium fuses with the falx cerebri, and its free edges bound the tentorial notch through which the Brainstem passes.

The falx cerebelli originates superiorly from the tentorium cerebelli, extends along the internal occipital crest, and reaches the foramen magnum, which it bounds with two limbs.

The diaphragm of the sella, diaphragma sellae, is a horizontal plate that covers the sella turcica from above, housing the Pituitary Gland. It forms an aperture for the infundibulum of the pituitary gland, through which the latter connects to the Hypothalamus.

Arteries, Veins, nerve fibers, and venous sinuses run within the dura mater.

The venous sinuses, sinus durae matris (Fig. 169 A, B), are spaces located between the layers of the dura mater in places where it attaches to the cranial bones. The walls of the sinuses are dense, endothelial-lined, and non-collapsible, which ensures an uninterrupted outflow of blood. Anatomical structures (trabeculae, septa, plates) have been found within the lumen of the sinuses, which, according to K. D. Balyasov (1950), regulate blood outflow. The following sinuses of the dura mater are distinguished:

- transverse sinus, sinus transversus;

- sigmoid sinus, sinus sigmoideus;

- superior sagittal sinus, sinus sagittalis superior;

- inferior sagittal sinus, sinus sagittalis inferior;

- straight sinus, sinus rectus;

- occipital sinus, sinus occipitalis;

- cavernous sinus, sinus cavernosus;

- superior and inferior petrosal sinuses, sinus petrosi superior et inferior.

The region where the transverse, superior sagittal, straight, and occipital sinuses meet forms a venous dilation known as the confluence of sinuses, confluens sinuum. This dilation corresponds to the cruciate eminence, eminentia cruciata, of the occipital bone.

The arachnoid mater of the brain, arachnoidea mater cranialis (Figs. 169, 170), is a semi-transparent, avascular connective tissue membrane that envelops the brain. The arachnoid mater does not penetrate into the depth of the sulci and fissures of the brain, but bridges across the gyri; consequently, the subarachnoid space, spatium subarachnoideum, filled with cerebrospinal fluid, is formed between it and the pia mater. This space is a capillary cleft that expands along the course of major cerebral vessels over the sulci and fissures of the brain. This space communicates with the cerebral ventricles through apertures located in the posterior wall of the Fourth ventricle, apertura mediana et lateralis ventriculi quarti^**. The subarachnoid space of the brain continues through the foramen magnum into the subarachnoid space of the Spinal Cord. In certain areas, the subarachnoid space expands beneath the arachnoid membrane to form subarachnoid cisterns, cisternae subarachnoidales. The following subarachnoid cisterns are distinguished:

- cerebellomedullary cistern, cisterna cerebellomedullaris, which communicates with the fourth ventricle through the median and lateral apertures;

- cistern of the lateral sulcus of the cerebrum, cisterna fossa lateralis cerebri. It contains the Branches of the middle cerebral artery and the insular arteries;

- the interpeduncular cistern, cisterna interpedunculars. It contains the Oculomotor nerve, the Basilar artery, the superior cerebellar arteries, and the posterior cerebral artery;

- the chiasmatic cistern, cisterna chiasmatis, which is located around the optic chiasm.

Subarachnoid spaces serve as reserve reservoirs that can accommodate various PARTS OF THE brain when its volume increases due to pathological conditions (B. S. Khomynsky, 1962). Such displacements most commonly occur in areas where the subarachnoid spaces (cisterns) are expanded, for example: herniation of the cerebellar Tonsils into the occipitocervical dural funnel, herniation of the parahippocampal gyrus beneath the edge of the tentorial notch, or herniation of the cingulate gyrus beneath the lower edge of the falx.

* Apertura mediana, formerly known as the foramen of Magendie;

** Apertura lateralis, formerly known as the foramen of Luschka.

Fig. 169 A. Dura mater of the brain, dura mater encephali (superior view) according to R. D. Sinelnikov

Fig. 169 B. Venous sinuses of the dura mater on the internal cranial base (according to F. Kiss and J. Szentágothai)

The arachnoid mater is connected to the pia mater by collagen and elastic fibers. Along with the cisterns, the arachnoid forms arachnoid granulations (granulationes arachnoidales)—specialized protrusions of the arachnoid mater that ensure the outflow of cerebrospinal fluid into the Venous system. They form depressions on the internal surface of the cranial bones known as the granular foveolae.

In addition to cisterns, other derivatives of the arachnoid mater include the denticulate ligaments, lig. denticulatum, and the choroid plexuses of the ventricles, plexus choroideus ventriculi. The following choroid plexuses are distinguished: the choroid plexus of the lateral ventricle, the choroid plexus of the Third ventricle, and the choroid plexus of the fourth ventricle. The choroid plexuses produce cerebrospinal fluid, liquor cerebrospinalis.

The pia mater, pia mater cranialis (Figs. 169, 170), is a connective tissue membrane that closely adheres to the brain surface, dipping into its sulci and fissures. It participates in The formation of the choroid plexuses of the brain ventricles, plexus choroideus, which produce cerebrospinal fluid. Together with the arachnoid, the pia mater covers the Cranial Nerves until they exit the cranial cavity.

Fig. 170. Arachnoid and pia mater of the brain, arachnoidea et pia mater encephali in sagittal section (medial surface)

Within the thickness of the pia mater lie the vascular networks of the brain, which penetrate into the cortex and White matter OF the hemispheres, providing their blood supply. The pia mater accompanies large and medium-sized arterial vessels, reaching down to the arterioles. Located between the wall of the arterial vessel and the pia mater are the Virchow-Robin spaces filled with cerebrospinal fluid. These spaces communicate with the subarachnoid space—situated between the arachnoid and pia maters, where cerebrospinal fluid circulates. Traversing the Virchow-Robin spaces across the Blood Vessels are collagen and elastic fibrils that anchor the vessels and help cushion them against pulsations.

Blood supply to the Meninges

The blood supply to the dura mater of the brain is provided by the following arteries: a. meningea media, a. maxillaris, a. meningea anterior et posterior, a. ethmoidalis anterior, and a. pharyngea ascendens. In addition to these arteries, meningeal branches (rr. meningei, a. lacrimalis, a. stylomastoidea, a. occipitalis, a. carotis internae, a. vertebralis) supply the dura mater.

These arteries and branches form a wide-meshed arterial network within the dura mater, from which smaller arteries branch off to form an arteriolocapillary-venular network.

Venous drainage is carried out by the meningeal veins (vv. meningeae anterior, media et posterior), which form a venous network within the dura mater.

No Lymphatic vessels have been found in the dura mater. The arachnoid mater of the brain is avascular. The pia mater features a well-developed capillary network derived from branches of the cerebral arteries. Venous drainage from the pia mater flows into the cerebral veins. Neither the arachnoid nor the pia mater contains lymphatic vessels.

Innervation of the Meninges

The innervation of the dura mater of the brain is provided by the V, VI, IX, X, XI, and XII cranial nerve pairs, as well as the three upper cervical sympathetic ganglia.

The innervation of the arachnoid and pia maters of the brain is supplied by branches of the III–XII cranial nerve pairs, as well as sympathetic branches from the internal carotid and vertebral plexuses, which form nerve plexuses around the cerebral arteries.