Chapter 11: Ventricles and Coverings of the Brain

Within the brain is a communicating system of cavities that are lined with ependyma and filled with cerebrospinal fluid (CSF): There are two lateral ventricles, the third ventricle (between the halves of the diencephalon), the cerebral aqueduct, and the fourth ventricle within the brain stem (Fig 11–1).

Lateral Ventricles and Choroid Plexus

The lateral ventricles are the largest. They each include two central portions (body and atrium) and three extensions (horns).

The choroid plexus is the site where cerebrospinal fluid (CSF) is produced. It is a fringe-like vascular process of pia mater containing capillaries of the choroid arteries. It projects into the ventricular cavity and is covered by an epithelial layer of ependymal origin (Figs 11–2 and 11–3). The attachment of the plexus to the adjacent brain structures is known as the tela choroidea. The choroid plexus extends from the interventricular foramen, where it joins with the plexuses of the third ventricle and opposite lateral ventricle, to the end of the inferior horn. (There is no choroid plexus in the anterior and posterior horns.)

The anterior (frontal) horn is in front of the interventricular foramen. Its roof and anterior border are formed by the corpus callosum; its vertical medial wall, by the septum pellucidum; and the floor and lateral wall, by the bulging head of the caudate nucleus.

The central part, or body, of the lateral ventricle extends from the interventricular foramen to a point opposite the splenium of the corpus callosum. Its roof is formed by the corpus callosum and its medial wall by the posterior portion of the septum pellucidum. The floor contains (from medial to lateral side) the fornix, the choroid plexus, the lateral part of the dorsal surface of the thalamus, the stria terminalis, the vena terminalis, and the caudate nucleus. The atrium, or trigone, is a wide area of the body that connects with the posterior and inferior horns (Fig 11–4).

The posterior (occipital) horn extends into the occipital lobe. Its roof is formed by fibers of the corpus callosum. On its medial wall is the calcar avis, an elevation of the ventricular wall produced by the calcarine fissure.

The inferior (temporal) horn traverses the temporal lobe, whose white substance forms its roof. Along the medial border are the stria terminalis and the tail of the caudate nucleus. The amygdaloid nuclear complex bulges into the upper terminal part of the inferior horn, whose floor and medial wall are formed by the fimbria, hippocampus, and collateral eminence.

The two interventricular foramens, or foramens of Monro, are apertures between the column of the fornix and the anterior end of the thalamus. The lateral ventricles communicate with the third ventricle through these foramens (see Fig 11–1).

Third Ventricle

The third ventricle is a narrow vertical cleft between the two halves of the diencephalon (see Figs 11–1, 11–2, 11–3, 11–4). The roof of the third ventricle is formed by a thin tela choroidea (a layer of ependyma) and pia mater from which a small choroid plexus extends into the lumen of the ventricle (see Fig 9–1). The lateral walls are formed mainly by the medial surfaces of the two thalami. The lower lateral wall and the floor of the ventricle are formed by the hypothalamus; the anterior commissure and the lamina terminalis form the rostral limit.

The optic recess is an extension of the third ventricle between the lamina terminalis and the optic chiasm. The hypophysis is attached to the apex of its downward extension, the funnel-shaped infundibular recess. A small pineal recess projects into the stalk of the pineal body. A large extension of the third ventricle above the epithalamus is known as the suprapineal recess.

Cerebral Aqueduct

The cerebral aqueduct is a narrow, curved channel running from the posterior third ventricle into the fourth. It contains no choroid plexus (see Figs 11–1 and 11–4).

Fourth Ventricle

The fourth ventricle is a pyramid-shaped cavity bounded ventrally by the pons and medulla oblongata (see Figs 7–14, 11–1, and 11–3); its floor is also known as the rhomboid fossa. The lateral recess extends as a narrow, curved extension of the ventricle on the dorsal surface of the inferior cerebellar peduncle. The fourth ventricle extends under the obex into the central canal of the medulla.

The incomplete roof of the fourth ventricle is formed by the anterior and posterior medullary vela. The anterior medullary velum extends between the dorsomedial borders of the superior cerebellar peduncles, and its dorsal surface is covered by the adherent lingula of the cerebellum. The posterior medullary velum extends caudally from the cerebellum. The point at which the fourth ventricle passes up into the cerebellum is called the apex, or fastigium.

The position of the cerebellum, just above the roof of the fourth ventricle, has important clinical implications. Mass lesions of the cerebellum (eg, tumors) or swelling of the cerebellum after a cerebellar infarction can compress the fourth ventricle, producing acute obstructive hydrocephalus.

The lateral aperture (foramen of Luschka) is the opening of the lateral recess into the subarachnoid space near the flocculus of the cerebellum. A tuft of choroid plexus is commonly present in the aperture. The medial aperture (foramen of Magendie) is an opening in the caudal portion of the roof of the ventricle. Most of the outflow of CSF from the fourth ventricle passes through this aperture, which varies in size.

The tela choroidea of the fourth ventricle is a layer of pia and ependyma that contains small vessels and lies in the posterior medullary velum. It forms the choroid plexus of the fourth ventricle.

Three membranes, or meninges, envelop the brain: the dura, the arachnoid, and the pia. The dura, the outer membrane, is separated from the thin arachnoid by a potential compartment, the subdural space, which normally contains only a few drops of CSF. An extensive subarachnoid space containing CSF and the major arteries separates the arachnoid from the pia, which completely invests the brain. The arachnoid and the pia, known collectively as leptomeninges, are connected by thin strands of tissue, the arachnoid trabeculae. The pia, together with a narrow extension of the subarachnoid space, accompanies the vessels deep into the brain tissue; this space is called the perivascular space, or Virchow–Robin's space.

Dura

The dura, which was formerly called the pachymeninx, is a tough, fibrous structure with an inner (meningeal) and an outer (periosteal) layer (Figs 11–4 and 11–5). (Most of the dura's venous sinuses lie between the dural layers.) The dural layers over the brain are generally fused, except where they separate to provide space for the venous sinuses and where the inner layer forms septa between brain portions. The outer layer is firmly attached to the inner surface of the cranial bones and sends vascular and fibrous extensions into the bone itself; the inner layer is continuous with the spinal dura.

One of the dural septa, the falx cerebri, extends like a curtain, down into the longitudinal fissure between the cerebral hemispheres (Figs 11–5 and 11–6). It attaches to the inner surface of the skull in midplane, from the crista galli to the internal occipital protuberance, where it becomes continuous with the tentorium cerebelli.

The tentorium cerebelli separates the occipital lobes from the cerebellum. It is a roughly transverse, shelflike membrane that attaches at the rear and side to the skull at the transverse sinuses; at the front, it attaches to the petrous portion of the temporal bone and to the clinoid processes of the sphenoid bone. Toward the midline, it fuses with the falx cerebri. The free, curved anterior border leaves a large opening, the incisura tentorii (tentorial notch), for passage of the upper brain stem, aqueduct, and vessels.

The falx cerebelli projects between the cerebellar hemispheres from the inner surface of the occipital bone to form a small triangular dural septum.

The diaphragma sellae forms an incomplete lid over the hypophysis in the sella turcica by connecting the clinoid attachments of the two sides of the tentorium cerebelli. The pituitary stalk passes through the opening in the diaphragma.

Arachnoid

The arachnoid, a delicate avascular membrane, covers the subarachnoid space, which is filled with CSF. The inner surface of the arachnoid is connected to the pia by fine arachnoid trabeculae (see Fig 11–5). The cranial arachnoid closely covers the inner surface of the dura mater but is separated from it by the subdural space, which contains a thin film of fluid. The arachnoid does not dip into the sulci or fissures except to follow the falx and the tentorium.

Arachnoid granulations consist of many microscopic villi (see Fig 11–5B). They have the appearance of berry-like clumps protruding into the superior sagittal sinus or its associated venous lacunae and into other sinuses and large veins. The granulations are sites of absorption of CSF.

The subarachnoid space between the arachnoid and the pia is relatively narrow over the surface of the cerebral hemisphere, but it becomes much wider in areas at the base of the brain. These widened spaces, the subarachnoid cisterns, are often named after neighboring brain structures (Fig 11–7). They communicate freely with adjacent cisterns and the general subarachnoid space.

The cisterna magna results from the bridging of the arachnoid over the space between the medulla and the cerebellar hemispheres; it is continuous with the spinal subarachnoid space. The pontine cistern on the ventral aspect of the pons contains the basilar artery and some veins. Below the cerebrum lies a wide space between the two temporal lobes. This space is divided into the chiasmatic cisterna above the optic chiasm, the suprasellar cistern above the diaphragma sellae, and the interpeduncular cistern between the cerebral peduncles. The space between frontal, parietal, and temporal lobes is called the cistern of the lateral fissure (cistern of Sylvius).

Pia

The pia is a thin connective tissue membrane that covers the brain surface and extends into sulci and fissures and around blood vessels throughout the brain (see Fig 11–5). It also extends into the transverse cerebral fissure under the corpus callosum. There it forms the tela choroidea of the third and lateral ventricles and combines with the ependyma and choroid vessels to form the choroid plexus of these ventricles. The pia and ependyma pass over the roof of the fourth ventricle and form its tela choroidea for the choroid plexus there.

Function

The CSF acts like a protective water jacket around the brain. It controls brain excitability by regulating the ionic composition, carries away metabolites (because the brain has no lymphatic vessels), and provides protection from pressure changes (venous volume versus CSF volume).

Composition and Volume

Normal CSF is clear, colorless, and odorless. Its more important normal values are shown in Table 11–1. Alterations in the composition of the CSF in various disorders are summarized in Chapter 24 and Table 24–1.

The CSF is present, for the most part, in a system that comprises two communicating parts. The internal portion of the system consists of two lateral ventricles, the interventricular foramens, the third ventricle, the cerebral aqueduct, and the fourth ventricle. The external part consists of the subarachnoid spaces and cisterns. Communication between the internal and external portions occurs through the two lateral apertures of the fourth ventricle (foramens of Luschka) and the median aperture of the fourth ventricle (foramens of Magendie). In adults, the total volume of CSF in all the spaces combined is normally about 150 mL. Between 400 and 500 mL of CSF is produced and reabsorbed daily.

Pressure

The normal mean CSF pressure is 70–180 mm of water; periodic changes occur with heartbeat and respiration. The pressure rises if there is an increase in intracranial volume (eg, with tumors or with some massive infarcts which cause brain swelling), blood volume (with hemorrhages), or CSF volume (with hydrocephalus), because the adult skull is a rigid box of bone that cannot accommodate the increased volume without a rise in pressure (Fig 11–9).

Circulation

Much of the CSF originates from the choroid plexuses within the lateral ventricles. The fluid passes through the interventricular foramens into the midline third ventricle; more CSF is produced here by the choroid plexus in the ventricle's roof (Fig 11–10). The fluid then moves through the cerebral aqueduct within the midbrain and passes into the rhombus-shaped fourth ventricle, where the choroid plexus adds more fluid. The fluid leaves the ventricular system through the midline and lateral apertures of the fourth ventricle and enters the subarachnoid space. From here it may flow over the cerebral convexities or into the spinal subarachnoid spaces. Some of it is reabsorbed (by diffusion) into the small vessels in the pia or ventricular walls. The remainder passes via the arachnoid villi into the venous blood (of sinuses or veins) in various areas, primarily over the superior convexity. There is normally a continuous circulation of CSF in and around the brain, in which production and reabsorption are in balance.

Several functionally important types of barriers exist in the nervous system, all of which play a role in maintaining a constant environment within and around the brain so that normal function continues and foreign or harmful substances are kept out. Some are readily visible, such as the three investing membranes (meninges), the dura, arachnoid, and pia (see Chapter 11); others are distinct only when examined with an electron microscope.

Blood–Brain Barrier

The blood–CSF barrier, the vascular–endothelial barrier, and the arachnoid barrier together form the blood–brain barrier. As noted in Chapter 2, capillary endothelial cells in most parts of the brain are joined by tight junctions that impede the diffusion of molecules out of or into the blood. This barrier is absent in several specialized regions: the basal hypothalamus, the pineal gland, the area postrema of the fourth ventricle, and several small areas near the third ventricle. Highly permeable fenestrated capillaries are present in these regions.

A. Blood–CSF Barrier

About 60% of the CSF is formed by active transport (through the membranes) from the blood vessels in the choroid plexus. Epithelial cells of the plexus, joined by tight junctions, form a continuous layer that selectively permits the passage of some substances but not others.

B. Vascular–Endothelial Barrier

Collectively, the blood vessels within the brain have a very large surface area that promotes the exchange of oxygen, carbon dioxide, amino acids, and sugars between blood and brain. Because other substances are kept out, the chemical composition of the extracellular fluid of the nervous system differs markedly from that of cell plasma. The blocking function is achieved by tight junctions between endothelial cells. There is evidence that neither the processes of astrocytes nor the basal laminas of endothelial cells prevent diffusion, even for molecules as large as proteins.

C. Arachnoid Barrier

Blood vessels of the dura are far more permeable than those of the brain; however, because the outermost layer of cells of the arachnoid forms a barrier, substances diffusing out of dural vessels do not enter the CSF of the subarachnoidal space. The cells are joined by tight junctions, and their permeability characteristics are similar to those of the blood vessels of the brain itself.

Ependyma

The ependyma lining the cerebral ventricles is continuous with the epithelium of the choroid plexus (Fig 11–14). Except for the ependyma of the lower third ventricle, most ependymal cells do not have tight junctions and cannot prevent the movement of macromolecules between ventricles and brain tissue.

Blood–Nerve Barrier

Large nerves consist of bundles of axons embedded in an epineurium. Each bundle is surrounded by a layer of cells called the perineurium; connective tissue within each bundle is the endoneurium. Blood vessels of the epineurium, which are similar to those of the dura, are permeable to macromolecules, but the endoneurial vessels, similar to those of the arachnoid, are not.

The skull (cranium), which is rigid in adults but pliable in newborn infants, surrounds the brain and meninges completely and forms a strong mechanical protection. In adults, the volume of the brain can increase beyond the capacity of the intact cranium as a result of swelling after injury, and this can further compress the already injured brain and cause herniation. Increased cranial pressure in infants may cause the fontanelles to bulge or the head to begin to enlarge abnormally (see Fig 11–11).

Essential structures (eg, cranial nerves, blood vessels) travel to and from the brain through various openings (fissures, canals, foramens) in the skull and are especially subject to compression as they traverse these small passageways. Thus, a good knowledge of their anatomy is important for the clinician.

Basal View of the Skull

The anterior portion of the base of the skull, the hard palate, projects below the level of the remainder of the inferior skull surface. The choanae, or posterior nasal apertures, are behind and above the hard palate. The pterygoid plates lie lateral to the choanae (Fig 11–15).

At the base of the lateral pterygoid plate is the foramen ovale, which transmits the third branch of the trigeminal nerve, the accessory meningeal artery, and (occasionally) the superficial petrosal nerve. Posterior to the foramen ovale is the foramen spinosum, which transmits the middle meningeal vessels. At the base of the styloid process is the stylomastoid foramen, through which the facial nerve exits.

The foramen lacerum is a large irregular aperture at the base of the medial pterygoid plate. Within its superior aspect is the carotid canal. The internal carotid artery, which emerges from this aperture, crosses only the superior part of the foramen lacerum.

Lateral to the foramen lacerum is a groove, the sulcus tubae auditivae, that contains the cartilaginous part of the auditory (eustachian) tube. It is continuous posteriorly with the canal in the temporal bone that forms the bony part of the auditory tube. Lateral to the groove is the lower orifice of the carotid canal, which transmits the internal carotid artery and the carotid plexus of the sympathetic nerves.

Behind the carotid canal is the large jugular foramen, which is formed by the petrous portion of the temporal and occipital bones and can be divided into three compartments. The anterior compartment contains the inferior petrosal sinus; the intermediate compartment contains the glossopharyngeal, vagus, and spinal accessory nerves; and the posterior compartment contains the sigmoid sinus and meningeal branches from the occipital and ascending pharyngeal arteries.

Posterior to the basilar portion of the occipital bone is the foramen magnum, which transmits the medulla and its membranes, the spinal accessory nerves, the vertebral arteries, and the anterior and posterior spinal arteries. The foramen magnum is bounded laterally by the occipital condyles.

Behind each condyle is the condyloid fossa, perforated on one or both sides by the posterior condyloid canal (which may transmit an emissary vein from the transverse sinus). Farther forward is the anterior condylar canal, or hypoglossal canal, which transmits the hypoglossal nerve and a meningeal artery.

Interior of the Skull

A. Calvaria

The inner surface of the calvaria (skull cap) is concave, with depressions for the convolutions of the cerebrum and furrows for the branches of the meningeal vessels. Along the midline is a longitudinal groove, narrow anteriorly and posteriorly wide, that contains the superior sagittal sinus. The margins of the groove provide attachment for the falx cerebri. At the rear are the openings of the parietal emissary foramens (when these are present). The sutures of the calvaria (sagittal, coronal, lambdoid, and others) are meshed lines of union between adjacent skull bones.

B. Floor of the Cranial Cavity

The internal, or superior, surface of the skull base forms the floor of the cranial cavity (Fig 11–16 and Table 11–2). It is divided into three fossae: the anterior fossa, the middle fossa, and the posterior fossa. The floor of the anterior fossa lies higher than the floor of the middle fossa, which in turn lies higher than the floor of the posterior fossa. A number of openings (many of them termed foramens) provide entrance and exit routes, through the floor of the cranial cavity, for vascular structures, cranial nerves, and the medulla.

1. Anterior cranial fossa

The floor of this is formed by the orbital plates of the frontal bone, the cribriform plates of the ethmoid, and the lesser wings and anterior part of the sphenoid. It is limited at the rear by the posterior borders of the lesser wings of the sphenoid and by the anterior margins of the chiasmatic groove.

The lateral segments of the anterior cranial fossa are the roofs of the orbital cavities, which support the frontal lobes of the cerebrum. The medial segments form the roof of the nasal cavity. The medial segments lie alongside the crista galli, which, together with the frontal crest, afford attachment to the falx cerebri.

The cribriform plate of the ethmoid bone lies on either side of the crista galli and supports the olfactory bulb. This plate is perforated by foramens for the olfactory nerves. The cranial openings of the optic canals lie just behind the flat portion of the sphenoid bone (planum sphenoidal).

2. Middle cranial fossa

This is deeper than the anterior cranial fossa and is narrow centrally and wide peripherally. It is bounded at the front by the posterior margins of the lesser wings of the sphenoid and the anterior clinoid processes. It is bounded posteriorly by the superior angles of the petrous portion of the temporal bones and by the dorsum sellae. It is bounded laterally by the temporal squamae and the greater wings of the sphenoid (Figs 11–16 and 11–17).

The narrow medial portion of the fossa presents the chiasmatic groove and the tuberculum sellae anteriorly; the chiasmatic groove ends on either side at the optic canal, which transmits the optic nerve and ophthalmic artery. Behind the optic canal, the anterior clinoid process is directed posteriorly and medially and provides attachment for the tentorium cerebelli. In back of the tuberculum sellae is a deep depression, the sella turcica; this structure, whose name means "Turkish saddle" (which it resembles), is especially important because it contains the hypophyseal fossa in which the hypophysis (pituitary) lies. The sella turcica is bounded posteriorly by a quadrilateral plate of bone, the dorsum sellae, whose sides project anteriorly as the posterior clinoid processes. These attach to slips of the tentorium cerebelli.

On either side of the sella turcica is the broad and shallow carotid groove, curving upward from the foramen lacerum to the medial side of the anterior clinoid process. This groove contains the internal carotid artery, surrounded by a plexus of sympathetic nerves.

The lateral segments of the middle fossa are deeper than its middle portion; they support the temporal lobes of the brain and show depressions that mark the convolutions of the brain. These segments are traversed by furrows for the anterior and posterior branches of the middle meningeal vessels, which pass through the foramen spinosum.

The superior orbital fissure is situated in the anterior portion of the middle cranial fossa. It is bounded above by the lesser wing, below by the greater wing, and in the middle by the body of the sphenoid. The superior orbital fissure transmits into the orbital cavity the oculomotor nerve, the trochlear nerve, the ophthalmic division of the trigeminal nerve, the abducens nerve, some filaments from the cavernous plexus of the sympathetic nerves, the ophthalmic veins, and the orbital branch of the middle meningeal artery.

The maxillary division of the trigeminal nerve passes through the foramen rotundum, which is located behind the medial wall of the superior orbital fissure. Behind the foramen rotundum, the foramen of Vesalius, transmits an emissary vein or a cluster of small venules; it can be large, small, multiple, or absent in different skulls. The foramen ovale, which transmits the mandibular division of the trigeminal nerve, the accessory meningeal artery, and the lesser superficial petrosal nerve, is posterior and lateral to the foramen rotundum.

The foramen lacerum is medial to the foramen ovale. Its inferior segment is filled by fibrocartilage. Its superior segment transmits the internal carotid artery, which is surrounded by a plexus of sympathetic nerves. The anterior wall of the foramen lacerum is pierced by the pterygoid canal.

3. Posterior cranial fossa

This fossa is larger and deeper than the middle and anterior cranial fossae. It is formed by the occipital bone, the dorsum sellae and clivus of the sphenoid bone, and portions of the temporal and parietal bones (see Fig 11–16).

The posterior fossa, or infratentorial compartment, contains the cerebellum, pons, medulla, and part of the midbrain. It is separated from the middle cranial fossa in and near the midline by the dorsum sellae of the sphenoid bone and on either side by the superior angle of the petrous portion of the temporal bone (petrous pyramid).

The foramen magnum lies in the center of the fossa. Just above the tubercle is the anterior condylar canal, or hypoglossal canal, which transmits the hypoglossal nerve and a meningeal branch for the ascending pharyngeal artery.

The jugular foramen lies between the lateral part of the occipital bone and the petrous portion of the temporal bone. The anterior portion of the foramen transmits the inferior petrosal sinus, the posterior portion transmits the transverse sinus and some meningeal branches from the occipital and ascending pharyngeal arteries, and the intermediate portion transmits the glossopharyngeal, vagus, and spinal accessory nerves.

Above the jugular foramen lies the internal acoustic meatus for the facial and acoustic nerves and the internal auditory artery. The inferior occipital fossae, which support the hemispheres of the cerebellum, are separated by the internal occipital crest, which serves for attachment of the falx cerebelli and contains the occipital sinus. The posterior fossae are surrounded by deep grooves for the transverse sinuses.

Tumors, inflammatory lesions, and other mass lesions can invade, and occlude, the foramens in the base of the skull. When they do so, they can compress and injure the cranial nerves and vessels running through these foramens. An example is shown in Figure 11–18.