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).
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).