Pia mater (soft inner layer of the dural membrane system)

The pia mater, which contains vessels, is the innermost layer of the three meningeal membranes. It consists of a thin layer of connective tissue with a lot of elastic fibers and is very close to the gyri of the brain substance itself, but is not fused with it. Vessels enter the brain from it. Furthermore, it creates the choroid plexus, a network of bold vessels, which enter the brain ventricles and produce the cerebrospinal fluid (CSF).

Arachnoidea (middle layer of the meningeal membranes)

The arachnoidea has a gauze-like spongy structure and can be differentiated into two layers. The outward layer beyond the dura mater is separated from it by a thin cleft, the subdural space, which contains some veins and nerves. The inner layer consists of a trabecular framework.

Between the arachnoidea and pia mater there is the subarachnoidal space. Both membranes are connected by the trabecular framework in the subarachnoidal space. It is filled with CSF and forms the outer CSF spaces. It is narrow at the apex of the skull, but because the arachnoidea follows the pia mater, larger caverns are created in areas where the brain and bony cranial wall are further apart, at the base of the cranium. These widened, CSF-filled spaces are called cisterns (cerebromedullar cistern, interpeduncular cistern, chiasmatic cistern, cisterna ambiens). Outgrowths of the arachnoidea, the arachnoidal villi, project into these spaces in the venous drainage of the inner skull, particularly that of the sagittal sinus. Through these villi the CSF can flow out into the venous system. The arachnoidea continues as the perineurium (nerve sheet) of the nerves passing through.

Dura mater (hard outer layer of the meningeal membranes)

The dura mater consists of dense, uneven, very strong connective tissue with a lot of collagenous fibers. It is very tight and not permeable to the CSF. A special layer of flat fibroblastic cells without extracellular collagen and extracellular space can be found at the transition between the dura and arachnoidea (Haines et al. 1993). This dural borderline can be divided into periostal dura and meningeal dura. There is no epidural space like that in the spine. The dura mater continues as the epineurium of efferent nerves that leave the skull. The skin of the scalp and the dura are connected by emissary veins, which may be under enormous tension. In the region of the ethmoid bone cells, the tegmentum and sinus sigmoideus, the dura is very flat.

The inner layer (dura meningealis) is weaker in its structure than the outer dura layer or the arachnoidea (Haines et al. 1993). The dura of an adult can resist stronger forces than that of a newborn (Dragoi 1995). According to Arbuckle (1994), this fibrous structure enables the intracranial dura and the intraspinal dura to transmit different forces. These “pathways for transmission of forces” find their way by the fibrous structures of the dura, the so-called “stress-fibers”. Arbuckle defines the following groups: horizontal, vertical, transversal, and circular. The direction of fibers in the dura mater cranialis can possibly be traced back to the results of mechanical forces during embryonic development, when collagenous fibers are brought into line by stress forces (Hamann et al. 1998). Between the dura periostale and dura meningeale there are some other important structures apart from the venous blood sinuses.

Horizontal and vertical dural system

The membranes inside the cranium are connected anatomically as well as functionally and so affect each other. Due to their different positions and orientations they can be divided into four septa: the falx cerebri, tentorium cerebella, falx cerebelli, and diaphragma sellae. Collagenous fiber bundles of the falx cerebri and falx cerebella form arcs in the anterior, intermediate, and posterior regions that cross each other perpendicularly. In the course of growth, fiber organization modulates from a 45° angle to a 90° angle (Dragoi 1995).

According to Delaire (1978), the horizontal system (tentorium cerebella and diaphragma sellae) acts as a clamp or tightener for the cranial base, while the vertical system (falx cerebri and falx cerebella) tightens the cranial vault. Delaire argues that the tension in the horizontal and vertical dural system is maintained and regulated by the continuous tone of the neck muscles and the sternocleidomastoid muscle, but this is controversial. According to Ferré et al. (1990), movement of the neck muscles can be transmitted via the aponeurotic part of the scalp. However, only very weak and secondary movement can be sensed there, even though the aponeurotic structure of the scalp is clearly movable, unlike the highly immobile falx cerebri and falx cerebelli. According to Sutherland (1939), tension in every part of the membranous system can have an influence at all other parts of that system due to the structural bond. The dural membranes safeguard the integrity of the cranium, especially in early childhood, by way of their attachment to the cranial bones, in case of exposure to force. Additionally, it is thought that involuntary “jointed” movements of similar cranial bones are regulated via synchronicity with the rhythm of the primary respiratory mechanism. Every difference in tension at one side of the membrane alters the complete unit and leads to a new balance.

The falx cerebri divides each cranial hemisphere from the other. Its anterior downward border attaches at the gallic crest of the ethmoid bone; continues across the foramen caecum, the frontal crest and the borders of the sulcus for the superior sagittal sinus of the frontal bone; then follows the parietal crest of the parietal bones, the sulcus sagittalis of the occipital bone, until it gets to the internal occipital protuberance. There the falx is involved in the formation of the straight sinus and at this structure both layers of the falx cerebri detach from each other and continue into the tentorium cerebelli. At the parietal bones it forms the sagittal sinus, while its free margin contains the inferior sagittal sinus.

The tentorium cerebelli (“La Tente”, Winslow 1732) divides the cerebrum and cerebellum from each other and stretches over the cerebellum like a tent. Above the tentorium there are, apart from the cerebral hemispheres, the subcortical nuclei and the thalamus. Like the falx cerebri and falx cerebella, the tentorium starts at the straight sinus and is attached there, too. The tentorium is attached posteriorly at the inner occipital protuberance and at both sides at the transversal groins of the internal occipital bone, where it forms the transverse sinus. Sideways, it leads along the sinus across the parietomastoideal suture and attaches for a short distance with its superior layer to the inferior back edge of the parietal bone. Meanwhile, the inferior layer can be found as an attachment to the mastoid process of the temporal bone. This is an important place, because its attachments continue from there along the mastoid process and the superior edge of the petrous part of the temporal bone, where it encloses the superior petrosal sinus. The inferior lateral layers are attached to the clinoid processes of the sphenoid bone. The free internal borders of the tentorium continue in an anterior direction, cross over the anterior inferior layers, and are attached to the anterior clinoid processes of the minor wings of the sphenoid bone. Here, where the internal branches of the tentorium cross the outer branches, the abducent nerve can be found. Obviously, the abducent nerve can be disturbed by tentorium tension. A large oval opening, the tentorial incisura, is occupied anteriorly by the midbrain and the interpeduncular cistern, and posteriorly by the rounded end (or splenium) of the corpus callosum.

Pia mater spinalis

This membrane contains vessels and nerves. From the pia mater, on both sides of the spinal cord, a connective tissue plate, the denticulate ligament, leads to the dura mater spinalis. It fixes the spinal cord and divides the two spinal nerve roots from each other. The pia mater follows the inner side of the spine and ends as a long, slender filament, the filum terminale, at the dorsal surface of the coccygeus bone.