Meninges

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Basics

Layers

The medulla spinalis and its membranes.

Dura mater

  • tough, fibrous, hard
  • outer and thickest meningeal layer
  • rests on the arachnoid
  • thin film of fluid separates the two layers
  • the potential space between the arachnoid and dura is the subdural space
  • In the cranium, the dura mater is adherent to the bones of the cranial cavity
  • In certain areas, dural folds project into the cranial cavity to separate different parts of the brain
  • In some areas, the dura contains venous sinuses
  • Meningeal blood also runs in the dura
    • Most important of these are the middle meningeal vessels, which enter and leave through the foramen spinosum
    • Middle meningeal artery is a branch of the maxillary artery

Arachnoid mater

  • spider-like
  • thin and transparent
  • sub-arachnoid space is filled with CSF
  • extends to S2 vertebral level (roughly)
  • thin strands of connective tissue cross the sub-arachnoid space, linking arachnoid to pia.
  • The roots of the spinal nerves cross the sub-arachnoid space to reach their intervertebral foramina

Pia mater

  • clinging mother
  • thin and transparent
  • adheres tightly to surface of spinal cord and to emerging nerve rootlets and roots.

Support structures

Denticulate ligament

  • toothlike strings
  • ribbon-like thickening of the pia mater
  • supports and suspends spinal cord
  • pin arachnoid to dura (while extending from pia to dura)

Filum terminale

  • thread at the end
  • derivative of the pia mater
  • goes from conus medullaris to anchor at the back of the coccyx

Specializations

Dural folds

Dura mater and its processes exposed by removing part of the right half of the skull and the brain.

Falx cerebri

  • Falx = sickle
  • a vertical fold, in the sagittal plane, passing between the two cerebral hemispheres
  • Its attached margin is continuous with the dura lining the cranial cavity
  • Anteriorly, it is attached to the crista galli (galli = cock's comb; crista = crest on a rooster's head) which is part of the ethmoid, and the frontal crest of the frontal bone
  • Posteriorly, it meets the tentorium cerebelli
  • Has a lower, free margin which also attaches anteriorly to the crista galli and posteriorly to the tentorium cerebelli
  • The corpus callosum passes under it

Tentorium cerebelli

  • tent-shaped sheet, in the horizontal plane, forming a roof for the posterior cranial fossa
  • Separates the cerebral hemispheres, above, from the cerebellum, below
  • There is a central opening in the anterior part of the tentorium (tentorial notch)
    • part of the brainstem passes through this notch
  • On each side, the "attached" margin attaches to the posterior clinoid process (projection of the dorsum sellae of the sphenoid), the petrous part of the temporal bone and the occipital bone
  • The "free" margin of the notch attaches anteriorly on each side to an anterior clinoid process (also a projection of the sphenoid)


Diaphragma sellae

Tentorium cerebelli seen from above.
  • a small, horizontal dural sheet between the anterior and posterior clinoid processes
  • Forms a roof for the pituitary fossa, which houses the pituitary gland
  • Has a small, central opening through which the pituitary stalk (infundibulum) passes, to reach the brain

Falx cerebelli

  • Falx = sickle
  • small, posterior, mid-line fold, extending inferiorly from the tentorium cerebelli
  • The fold partially separates the two cerebellar hemispheres

Venous sinuses

Dural Venous Sinuses

  • endothelial-lined, valveless vessels
  • located within the dura mater
  • Receive venous blood from the brain via cerebral veins
  • Communicate with extracranial veins (e.g., on the scalp) via emissary veins.

Superior sagittal sinus

  • located in the fixed margin of falx cerebri
  • Has lateral extensions (lacunae laterales) into which tufts of arachnoid (arachnoid granulations) project.
  • Widens posteriorly and usually turns to the right at the junction between the falx cerebri and tentorium cerebelli, to form the right transverse sinus

Inferior sagittal sinus

Straight sinus

  • passes posteriorly in the junction between the falx cerebri and tentorium cerebelli
  • Usually turns to the left to become the left transverse sinus

Transverse sinuses

  • Right and Left
  • Located in the attached margin of the tentorium
  • pass forward to the posterior borders of the petrous part of the temporal bones
  • Leave the tentorium and pass inferiorly as the sigmoid sinuses
  • The two transverse sinuses may communicate in the vicinity of the internal occipital protuberance, at the confluence of sinuses

Sigmoid sinuses

  • each pursues an S-shaped course within the dura, along the lateral wall of the posterior cranial fossa
  • Then, they pass through the jugular foramen to enter the internal jugular vein

Cavernous sinuses

  • very important because of their venous connections and because of the structures related to them
  • Located on each side of the pituitary fossa and communicate with each other around the pituitary gland (circular sinus)
  • Also communicate with ophthalmic veins of the orbit, with the sigmoid sinus via the superior petrosal sinus, and with the internal jugular vein via the inferior petrosal sinus

Occipital sinus

  • passes in the attached part of the falx cerebelli, from the confluence of sinuses, inferiorly, to the region of the foramen magnum, where it communicates with vertebral veins.

Arachnoid granulations

  • sites of drainage of C.S.F. into the venous system

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