Atlas of Anatomy. Head and Neuroanatomy. Michael Schuenke

12. Meningesof the Brain and Spinal Cord

12.1 Brain and Meninges in situ

A Brain and meninges in situ

Superior view, a The calvaria has been removed, and the superior sagittal sinus and its lateral lacunae have been opened; b The dura mater has been removed from the left hemisphere, and the dura and arachnoid have been removed from the right hemisphere.

The brain and spinal cord are covered by membranes called meninges, which form a sac filled with cerebrospinal fluid. The meninges are composed of the following three layers:

 Outer layer: The dura mater (often shortened to “dura”) is a tough layer of collagenous connective tissue. It consists of two layers, an inner meningeal layer and an outer endosteal layer. The periosteal layer adheres firmly to the periosteum of the calvaria within the cranial cavity, but it is easy to separate the inner layer from the bone in this region, leaving it on the cerebrum as illustrated here (a).

 Middle layer: The arachnoid (arachnoid membrane) is a translucent membrane through which the cerebrum and the blood vessels in the subarachnoid space can be seen (b).

 Inner layer: The pia mate г directly invests the cerebrum and lines its fissures (b).

The arachnoid and pia are collectively called the leptomeninges. The space between them, called the subarachnoid space, is filled with cerebrospinal fluid and envelops the brain (see C, p. 191). It contains the major cerebral arteries and the superficial cerebral veins, which drain chiefly through “bridging veins" into the superior sagittal sinus. The dura mater in the midline forms a double fold between the periosteal and meningeal layers that encloses the endothelium-lined superior sagittal sinus (see B, p.254), which has been opened in the illustration. Inspection of the opened sinus reveals the arachnoid granulations (Pacchionian granulations, arachnoid villi). These protrusions of the arachnoid are sites for the reabsorption of cerebrospinal fluid (see A, p. 194). Arachnoid granulations are particularly abundant in the lateral lacunae of the superior sagittal sinus. The dissection in a shows how the middle meningeal artery is situated between the dura and calvaria. Rupture of this vessel causes blood to accumulate between the bone and dura, forming an epidural hematoma (see p.262).

В Projection of important brain structures onto the skull

Anterior view. The largest structures of the cerebrum (telencephalon) are the frontal and temporal lobes. The falx cerebri separates the two cerebral hemispheres in the midline (not visible here). In the brainstem, we can identify the pons and medulla oblongata on both sides of the midline below the telencephalon. The superior sagittal sinus and the paired sigmoid sinuses can also be seen. The anterior horns of the two lateral ventricles are projected onto the forehead.

C Projection of important brain structures onto the skull

Left lateral view. The relationship of specific lobes of the cerebrum to the cranial fossae can be appreciated in this view. The frontal lobe lies in the anterior cranial fossa, the temporal lobe in the middle cranial fossa, and the cerebellum in the posterior cranial fossa. The following dural venous sinuses can be identified: the superior and inferior sagittal sinus, straight sinus, transverse sinus, sigmoid sinus, cavernous sinus, superior and inferior petrosal sinus, and occipital sinus.

12.2 Meninges and Dural Septa

A Brain in situ with the dura partially dissected from the arachnoid

Viewed from upper left. The dura has been opened and reflected upward, leaving the underlying arachnoid and pia mater on the brain. Because the arachnoid is so thin, we can see the underlying subarachnoid space and the vessels that lie within it (see C). The subarachnoid space no longer contains cerebrospinal fluid at this stage of the dissection and is therefore collapsed. Before the superficial cerebral veins terminate in the sinus, they leave the subarachnoid space for a short distance and course between the neurothelium of the arachnoid and the meningeal layer of the dura to the superior sagittal sinus. These segments of the cerebral veins are called bridging veins (see C). Some of the bridging veins, especially the inferior cerebral veins, open into the transverse sinus. Injury to the bridging veins leads to subdural hemorrhage (see pp. 191 and 262).

В Dural septa

Left anterior oblique view. The brain has been shelled out of its cavity to demonstrate the dural septa. The falx cerebri appears as a fibrous sheet that arises from the crista galli of the ethmoid bone and separates the two cerebral hemispheres. At its site of attachment to the calvaria, the falx cerebri expands to accommodate the superior sagittal sinus. Additional septa are the tentorium cerebelli and falx cerebelli (not shown here). The tentorium cerebelli fans out into the groove between the cerebrum and cerebellum, while the falx cerebelli separates the two hemispheres of the cerebellum. Its root transmits the occipital sinus. Because the dural septa are rigid structures, portions of the brain may herniate beneath theirfree edges (see D). The brainstem passes through an opening in the tentorium cerebelli called the tentorial notch.

C Relationship of the meninges to the calvarium

a Coronal section through the vertex of the skull, anterior view. The endosteal layer of the dura mater and the periosteum of the skull are fused together (the periosteal layer of the dura mater), each layer consisting of a tough meshwork of fibrous tissue. At some sites the dura forms septa that dip into the fissures separating different brain regions. In the vertex region pictured here, the septum consists of the falx cerebri (other septa are shown in B). Located within the dura, between its endosteal and meningeal layers, are the principal venous channels of the brain, the dural venous sinuses (e.g., the superior sagittal sinus). Their walls are composed of dura and endothelium. Arachnoid granulations protrude from the subarachnoid space into the superior sagittal sinus. These projections are channels through which cerebrospinal fluid from the subarachnoid space can be reabsorbed by the venous system (details on p. 194 f). They can produce pits in the inner table of the skull (granular foveolae, see p.8). A schematic close-up (b) shows the relationship of the pia-arachnoid, which contains the slit-like subarachnoid space. This space is subdivided by arachnoid trabeculae that extend from the outer layer (arachnoid) to the inner layer (pia mater). At its boundary with the dura, the arachnoid is covered by flat cells which, unlike other meningeal cells, are joined together by “tight junctions" (neurothelium) to create a diffusion barrier between the blood and cerebrospinal fluid (see p. 196).

D Potential sites of brain herniation beneath the free edges of the meninges

Coronal section, anterior view. The tentorium cerebelli divides the cranial cavity into a supratentorial and an infratentorial space. The telencephalon is supratentorial, and the cerebellum is infratentorial (a). Because the dura is composed of tough, collagenous connective tissue, it creates a rigid intracranial framework. As a result, a mass lesion within the cranium may displace the cerebral tissue and cause portions of the cerebrum to become entrapped (herniate) beneath the rigid dural septa (= duplication of the meningeal layer of the dura).

a Axial herniation. This type of herniation is usually caused by generalized brain edema. It is a symmetrical herniation in which the middle and lower portions of both temporal lobes of the cerebrum herniate down through the tentorial notch, exerting pressure on the upper portion of the midbrain (bilateral uncal herniation).

If the pressure persists, it will force the cerebellar tonsils through the foramen magnum and also compress the lower part of the brainstem (tonsillar herniation). Because respiratory and circulatory centers are located in the brainstem, this type of herniation is life-threatening (see p. 231). Concomitant vascular compression may cause brainstem infarction, b Lateral herniation. This type is caused by a unilateral mass effect (e.g., from a brain tumor or intracranial hematoma), as illustrated here on the right side. Compression of the ipsilateral cerebral peduncle usually produces contralateral hemiparesis. Sometimes, the herniating mesiobasal portions of the temporal lobe press the opposite cerebral peduncle against the sharp edge of the tentorium. This damages the pyramidal tract above the level of its decussation, causing hemiparesis to develop on the side opposite the injury.

12.3 Meninges of the Brain and Spinal Cord

A Blood supply of the dura mater

Midsagittal section, left lateral view with branches of the middle meningeal artery exposed at several sites. Most of the dura mater in the cranial cavity receives its blood supply from the middle meningeal artery, a terminal branch of the maxillary artery. The other vessels shown here are of minor clinical importance. The essential function of the middle meningeal artery is, however, not to supply the meninges (as its name might suggest) but to supply the calvaria. Head injuries may cause the middle meningeal artery to rupture, leading to life-threatening complications (epidural hematoma; see C and pp. 189 and 262).

В Innervation of the dura mater In the cranial cavity (after von Lanz and Wachsmuth)

Superior view with the tentorium cerebelli removed on the right side. The intracranial meninges are supplied by meningeal branches from all three divisions of the trigeminal nerve and also by branches of the vagus nerve and the first two cervical nerves. Irritation of these sensory fibers due to meningitis is manifested clinically by headache and reflex nuchal stiffness (the neck is hyperextended in an attempt to relieve tension on the inflamed meninges). The brain itself is insensitive to pain.

C Meninges and their spaces

Transverse section through the calvaria (schematic). The meninges have two spaces that do not exist under normal conditions, as well as one physiological space:

 Epidural space: This space is not normally present in the brain (contrast with E, which shows the physiological epidural space in the spinal canal). It develops in response to bleeding from the middle meningeal artery or one of its branches (arterial bleeding). The extravasated blood separates the dura mater from the bone, dissecting an epidural space between the inner table of the calvaria and the dura (epidural hematoma, see p. 262).

 Subdural space: Bleeding from the bridging veins artificially opens the subdural space between the meningeal layer of the dura mater and upper layer of the arachnoid membrane (subdural hematoma, see p.262). The cells of the uppermost layer of the arachnoid (neurothelium) are interconnected by a dense network of tight junctions, creating a tissue barrier (blood-cerebrospinal fluid barrier).

• Subarachnoid space: This physiologically normal space lies just beneath the arach noid. It is filled with cerebrospinal fluid and is traversed by blood vessels. Bleeding into this space (subarachnoid hemorrhage) is usually arterial bleeding from an aneurysm (abnormal circumscribed dilation) of the basal cerebral arteries (see p. 262).

D Transverse section through the spinal cord and its meninges

Cervical vertebra viewed from above. Caudal to the foramen magnum, the dura mater separates from the periosteum; i.e., the meningeal and periosteal layers of the dura mater separate from each other to define a physiological space, the epidural space. This space is occupied by fatty tissue and venous plexuses. The dorsal and ventral roots of the spinal nerves course within the dural sac of the spinal cord and collectively form the cauda equina in the lower part of the sac (not shown here). The dorsal and ventral roots unite within a dural sleeve at the intervertebral foramina to form the spinal nerves. After the two roots have fused lateral to the spinal ganglion, the spinal nerve emerges from the dural sac. The pia mater invests the surfaces of the brain and spinal cord in the same fashion. The denticulate ligaments are sheets of pial connective tissue that pass from the spinal cord to the dura and are oriented in the coronal plane.

E Meninges in the cranial cavity and spinal canal

The periosteum of the bones and the meningeal layer of the dura mater are fused together inside the cranial cavity. Caudal to the foramen magnum, however, these two layers of collagenous connective tissue separate from each other to form the epidural space. Due to the mobility of the spinal column, the periosteum of the vertebrae must be free to move relative to the dural sac. This is accomplished by the presence of the epidural space, which exists physiologically only within the spinal canal. It contains fat and venous plexuses (see D). This space has major clinical importance, as it is the compartment into which epidural anesthetics are injected.