Meninges - Brain

Meninges

The brain is surrounded by mesodermal coverings, the meninges. The outer layer is the tough pachymeninx, or dura mater (A1). The inner layer is the soft leptomeninx which consists of two sheets, the arachnoidmater (A2) and the pia mater (A3).

Dura Mater (B)

The dura mater lines the inner surface of the skull and also forms the periosteum. Sturdy septa extend from it deep into the cranial cavity. A sickle-shaped fold of the dura, the falx of the cerebrum (B4), suspends verti-cally between the two cerebral hemi-spheres. It is attached rostrally to the crista galli and extends over the frontal crest to the internal occipital protuberance, where it turns into the tentorium of the cerebellum(B5) spanning both sides. The falx divides the superior part of the cranial cavity in such a way that each hemisphere is sup-ported in its own space. The tentorium of the cerebellum stretches like a tent across the cerebellum, lying in the posterior cranial fossa. It is attached along the trans-verse sulcus of the occipital bone and the upper margin of the petrous bone, leaving rostrally a wide opening for the passage of the brain stem (B6). At the lower surface of the tentorium and along the occipital crest, the falx of the cerebellum projects into the posterior cranial fossa.

The large venous channels, the sinuses of thedura mater, are embedded between the twosheets of the dura mater (see vol. 2). The di-agram shows cross sections of the superiorsagittal sinus (B8) and the transverse sinus(B9).

Certain structures are encapsulated by dural pockets and thus separated from the rest of the inner cavity. The sellar diaphragm (B10) spans the sella turcica and contains an aper-ture for passage of the hypophysial stalk, the diaphragmatic hiatus (B11). The trigeminalganglion (semilunar ganglion, Gasser’s gan-glion) on the anterior surface of the petrous bone is enclosed by a dural pocket, the trigeminal cave (Meckel’s space).

Arachnoid Mater (A)

The arachnoid mater (A2) adjoins closely the inner surface of the dura mater and is sepa-rated from it only by a capillary cleft, the subdural space (A12). It encloses the sub-arachnoid space (A13), which containscerebrospinal fluid and is connected with the pia mater by trabeculae (A14) and septae that form a dense meshwork and create a system of communicating cham-bers.Peduncled mushroomlike vegetations of the arachnoid protrude into the large sinuses, the arachnoid granulations (meningealgranules, or pacchionian bodies) (A15). Theyconsist of an arachnoid meshwork and are covered by mesothelium. The dura mater, which encloses them, is reduced to a mem-brane. These arachnoid villi are most abun-dant in the vicinity of the superior sagittal sinus (A16) and at the lateral lacunae (A17), and less frequent at the exits of spinal nerves. The CSF is absorbed into the venous blood in the area of the granulations. In older people, the granulations may also penetrate into the bone (formation of granular foveolae) (A18) and invaginate into the diploic veins.

Pia Mater (A)

The pia mater (A3) is the meningeal cover-ing that contains the blood vessels. It borders directly on the brain and forms the me-sodermal side of the pia–glia barrier. From here vessels enter the brain, and they are surrounded by the pia mater for some dis-tance (pial funnel).

A19 Scalp.

A20 Skull.

A21 Diploë