Meninges

Meninges

CHAPTER OBJECTIVES

To learn the structure and function of the three meninges that surround the brain and spinal cord.

To understand the venous sinuses within the skull and see how the meninges contribute to their walls.

A forty-four-year-old woman is seen by a neurologist because she is experiencing intense pain in the right eye. On physical examination, she is found to have a slight medial strabismus of the right eye, and the right pupil is smaller than normal. Further examination reveals numbness over the right cheek. A computed tomography scan shows the presence of an aneurysm of the right internal carotid artery within the cavernous sinus. The aneurysm is about the size of a pea.

The location of the carotid aneurysm within the cavernous sinus explains the ocular pain; pressure on the right abducens nerve is responsible for the paralysis of the

BRAIN MENINGES

The brain in the skull is surrounded by three protective membranes or meninges: the dura mater, the arachnoid mater, and the pia mater.

Dura Mater, dense fibrous membrane, main barrier.

The dura mater of the brain is conventionally described as two layers: the endosteal layer and the meningeal layer. These are closely united except along certain lines where they separate to form venous sinuses.

periosteal layer.

The endosteal layer is nothing more than the periostream covering the inner surface of the skull bones. At the foramen magnum, it does not become continuous with the dura mater of the spinal cord. Around the margins of all the foramina in the skull, it becomes continuous with the periosteum on the outside of the skull bones. At the sutures, it is continuous with the sutural ligaments. It is most strongly adherent to the bones over the base of the skull.

To appreciate the relationship of the meninges to the different forms of cerebral hemorrhage.

lateral rectus muscle producing the medial strabismus. The small pupil of the right eye is caused by the aneurysm pressing on the sympathetic plexus surrounding the carotid artery and producing paralysis of the dilator pupillae muscle. The numbness over the right cheek is due to pressure of the aneurysm on the right maxillary division of the trigeminal nerve as it passes forward through the lateral wall of the sinus.

This patient illustrates the necessity of knowing the relationships between the structures within the skull, especially in regions like the cavernous sinus, where so many important neural structures lie close to one another.

forms like a cult in spinal cord, nerves forms and dural fold.

The meningeal layer is the dura mater proper. It is a dense, strong fibrous membrane covering the brain and is continuous through the foramen magnum with the dura mater of the spinal cord. It provides tubular sheaths for the cranial nerves as the latter pass through the foramina in the skull. Outside the skull, the sheaths fuse with the epineurium of the nerves.

dural folds and reflections.

The meningeal layer sends inward four septa, the falx cerebri, falx cerebelli, tentorium cerebelli, and diaphragma sellae, which divide the cranial cavity into freely communicating spaces that lodge the subdivisions of the brain. The function of these septa is to restrict the displacement of the brain associated with acceleration and deceleration when the head is moved.

The falx cerebri is a sickle-shaped fold of dura mater that lies in the midline between the two cerebral hemispheres. Its narrow anterior end is attached to the internal frontal crest and the crista galli. Its broad posterior part blends in the midline with the upper surface of the

Falx cerebri, separates cerebral hemispheres. Tentorium cerebelli, separates cerebrum from cerebellum. Falx cerebelli, between cerebellar hemispheres. Diaphragma sellae, root over sella turcica, pituitary.

tentorium cerebelli. The superior sagittal sinus runs in its upper fixed margin, the inferior sagittal sinus runs in its lower concave free margin, and the straight sinus runs along its attachment to the tentorium cerebelli.

The tentorium cerebelli is a crescent-shaped fold of dura mater that roofs over the posterior cranial fossa. It covers the upper surface of the cerebellum and supports the occipital lobes of the cerebral hemispheres. In the anterior edge, a gap, the tentorial notch, allows passage of the midbrain,

which produces an inner free border and an outer attached or fixed border. The fixed border is attached to the posterior clinoid processes, the superior borders of the petrous bones, and the margins of the grooves for the transverse sinuses on the occipital bone. The free border runs forward at its two ends, crosses the attached border, and is affixed to the anterior clinoid process on each side. At the point where the two borders cross, the third and fourth cranial nerves pass forward to enter the lateral wall of the cavernous sinus.

Close to the apex of the petrous part of the temporal bone, the lower layer of the tentorium is pouched forward beneath the superior petrosal sinus to form a recess for the trigeminal nerve and the trigeminal ganglion.

The falx cerebri and the falx cerebelli are attached to the upper and lower surfaces of the tentorium, respectively. The straight sinus runs along its attachment to the falx cerebri, the superior petrosal sinus runs along its attachment to the petrous bone, and the transverse sinus runs along its attachment to the occipital bone.

The falx cerebelli, a small, sickle-shaped fold of dura mater attached to the internal occipital crest, projects forward between the two cerebellar hemispheres. Its posterior fixed margin contains the occipital sinus.

The diaphragma sellae is a small, circular fold of dura mater that forms the roof for the sella turcica. A small opening in its center allows passage of the stalk of the hypophysis cerebri.

infundibulum.