The meninges[1] of the encephalon are: one[2] and two leptomeninges[3]. They surround the encephalon and cranial nerve roots. They derive from the neural crest and the mesoderm. According to Lievre, the neural crest participates in the formation of the meninges of the forebrain but not in the meninges of the midbrain, hindbrain and spinal cord. These are derived only from the mesoderm (Page 77 of "The neural crest" by Le Douarin, N. 1982).


The pachymeninx is the dura mater[4]. It consists of an outer layer fixed to the periosteum, and an inner layer that forms extensions that separate parts of the encephalon. These extensions are: the falx[5] cerebri (that extends into the longitudinal fissure), the membranous tentorium[6] cerebelli (that extends between the cerebrum and the cerebellum) and the diaphragm sellae (that projects over the dorsum of the pituitary fossa forming an incomplete septum around the pituitary stalk). At the level of the hypophysis, the space between the outer and the inner layers of the dura mater is occupied by veins that drain to the cavernous and intercavernous sinuses.


The leptomeninges are: the arachnoid[7] and the pia mater[8]. As the two meninges derive from a mesenchyme layer that undergoes cavitation, the two meninges are called pia-arachnoid. The arachnoid and pia mater are held together by thin trabeculae of collagen fibers and fibroblasts. The arachnoid is covered by the dura mater and the pia mater covers the nervous parenchyma.


The space between the arachnoid and the pia mater is the subarachnoid space. It contains cerebrospinal fluid (CSF) and blood capillaries. In certain areas, there are enlargements called cisterns. The biggest cisterns are: the interpeduncular cistern (located between the crus cerebri), the chiasmatic cistern (around the optic chiasm), the quadrigeminal cistern (dorsal to the mesencephalic colliculi) and the cisterna magna or cerebellomedullary cistern (located between the cerebellum and the medulla oblongata).


The velum interpositum is a triangular double layer of pia mater formed by an invagination of the pia located between the thalamus and the fornix. Within the velum interpositum, there are two arachnoid layers: the dorsal envelops the internal cerebral veins, and the ventral is an extension of the arachnoid envelope of the pineal region. The velum interpositum forms the roof of the third ventricle and contains the choroid plexuses of the third ventricle and the deep cerebral veins. It extends caudally to give rise to the quadrigeminal cistern. This an enlarged portion of the subarachnoid space located immediately superior to the tectum of the mesencephalon. It contains the great cerebral vein (of Galen), the distal part of the quadrigeminal artery, a segment of the posterior cerebral artery, and the exit of the trochlear nerve.


[1] From the Greek plural meninx, “membrane”.

[2] From the Greek pachys, "thick".

[3] From the Greek lepto, "weak".

[4] From the Greek dura mater, "hard mother".

[5] From the Latin falx, "stickle".

[6] From the Latin tentorium, "tent".

[7] From the Greek arachne, "cobweb".

[8] From the Latin pia mater, "soft mother".