Dura mater, simply called the dura, is the outermost of the three meninges surrounding the brain and the spinal cord. The other two layers are arachnoid mater and pia mater.
The dura mater is described as composed of two layers: the endosteal layer and the meningeal layer. These layers are closely united except at places where they separate to form the venous sinuses.
Endosteal layer of dura mater:
This layer is nothing more than the ordinary periosteum covering the inner surface of the skull bones. One important fact to remember is that this layer does not extend through the foramen magnum to become continuous with the dura mater of the spinal cord. Thus spinal dura mater has no endosteal layer.
It provides tubular sheath for the cranial nerves as they pass through the various foramina of the skull. However, the sheath extends for only a small distance beyond the foramina and eventually fuses with the epineurium of the nerves.
Meningeal layer of dura mater:
It is a dense, strong and fibrous membrane that is the dura mater proper. It passes through the foramen magnum to be continuous with the dura mater of spinal cord.
The meningeal layer also provides tubular sheaths for the cranial nerves, when they exit through the foramina of the skull. Outside the skull, the sheaths fuse with the epineurium of the nerves.
Dural folds (Septa):
The meningeal layer of dura mater sends inward four septa (dural folds) that divide the cranial cavity into freely communicating spaces lodging the subdivisions of the brain. The function of these septa is to restrict the rotatory displacement of the brain. These folds are:
It is a sickle shaped fold of dura mater that lies in the midline between the two cerebral hemispheres. Its front end is narrow and is attached to the internal frontal crest and crista galli. Its posterior end is broad, which blends with the upper surface of the tentorium cerebelli.
In the upper fixed margin of the falx cerebri runs the superior sagittal sinus while in the lower free margin runs the inferior sagittal sinus. The straight sinus runs along its attachment to the tentorium cerebelli.
It is a crescent shaped fold of the dura mater that forms a roof over the posterior cranial fossa. It covers the upper surface of the cerebellum and supports the occipital lobes of the cerebral hemispheres.
On the anterior aspect of the tentorium cerebelli, there is a gap, known as the tentorial notch, for the passage of the midbrain. From the above description, it can be concluded that the tentorium cerebelli has an inner free border and outer fixed border. The fixed border is attached to the posterior clinoid processes and the margins of the grooves for transverse sinus on the occipital bone. The free margin, after crossing beneath the fixed margin, is attached to the anterior clinoid process on each side.
Three sinuses are related to the tentorium cerebelli as described below:
- Straight sinus: It runs along the attachment of the tentorium to the falx cerebelli.
- Superior petrosal sinus: It runs along the attachment of the tentorium to the petrous bone.
- Transverse sinus: It runs along the attachment of the tentorium to the occipital bone.
It is a small, sickle-shaped fold of dura that projects between the two hemispheres of cerebellum. It is attached anteriorly to the internal occipital crest and its posterior free margin contains the occipital sinus.
It is a small circular fold of dura that forms the roof for the sella turcica. It has a small opening in its center that allows the passage of the stalk of the pituitary gland.
Dura mater receives its nerve supply from branches of the trigeminal, vagus and first three cervical nerves. Branches from the sympathetic system also innervate it.
It is important to know that dura mater is sensitive to stretch and it is this very stimulus that produces the sensation of headache.
Dura mater has distinct arterial supply and venous drainage so it is best to discuss them separately.
Arterial supply of dura mater:
Dura mater receives arterial supply from branches of numerous arteries including the following:
- Internal carotid artery
- Maxillary artery
- Ascending pharyngeal artery
- Occipital artery
- Vertebral arteries
However, the most important, from clinical point of view, is middle meningeal artery, which is commonly damaged in head injuries.
The venous drainage of dura mater occurs by way of meningeal veins that lie in the endosteal layer of it. They follow the branches of the middle meningeal artery and eventually drain into the pterygoid venous plexus or the sphenoparietal sinus.