LATERAL VENTRICLE

Viewed from its lateral aspect, the lateral ventricle has a roughly C-shaped profile. The shape is a consequence of the developmental expansion of the frontal, parietal and occipital regions of the hemisphere, which displaces the temporal lobe inferiorly and anteriorly. Both the caudate nucleus and the fornix, which lie in the wall of the ventricle, have adopted a similar morphology, so that the tail of the caudate nucleus encircles the thalamus in a C-shape, and the fornix traces the outline of the ventricle forwards to the interventricular foramen.

The lateral ventricle is customarily divided into a body and anterior (frontal), posterior (occipital) and inferior (temporal) horns. The anterior horn lies within the frontal lobe. It is bounded anteriorly by the posterior aspect of the genu and rostrum of the corpus callosum, and its roof is formed by the anterior part of the body of the corpus callosum. The anterior horns of the two ventricles are separated by the septum pellucidum. The coronal profile of the anterior horn is roughly that of a flattened triangle in which the rounded head of the caudate nucleus forms the lateral wall and floor (Figs 16.2, 16.3). The anterior horn extends back as far as the interventricular foramen. The body lies within the frontal and parietal lobes and extends from the interventricular foramen to the splenium of the corpus callosum. The bodies of the lateral ventricles are separated by the septum pellucidum, which contains the columns of the fornices in its lower edge. The lateral wall of the body of the ventricle is formed by the caudate nucleus superiorly and the thalamus inferiorly. The boundary between the thalamus and caudate nucleus is marked by a groove (Fig. 16.2) which is occupied by a fascicle of nerve fibres, the stria terminalis, and by the thalamostriate vein (see Fig. 22.5). The inferior limit of the body of the ventricle and its medial wall are formed by the body of the fornix (Fig. 16.4). The fornix is separated from the thalamus by the choroidal fissure. The choroid plexus occludes the choroidal fissure and covers part of the thalamus and fornix. The body of the lateral ventricle widens posteriorly to become continuous with the posterior and inferior horns at the collateral trigone or atrium. The posterior horn curves posteromedially into the occipital lobe. It is usually diamond-shaped or square in outline, and the two sides are often asymmetrical. Fibres of the tapetum of the corpus callosum separate the ventricle from the optic radiation, and form the roof and lateral wall of the posterior horn. Fibres of the splenium of the corpus callosum (forceps major) pass medially as they sweep back into the occipital lobe, and produce a rounded elevation in the upper medial wall of the posterior horn. Lower down, a second elevation, the calcar avis, corresponds to the deeply infolded cortex of the anterior part of the calcarine sulcus. The inferior horn is the largest compartment of the lateral ventricle and extends forwards into the temporal lobe. It curves round the posterior aspect of the thalamus (pulvinar), passes downwards and posterolaterally and then curves anteriorly to end within 2.5 cm of the temporal pole, near the uncus. Its position relative to the surface of the hemisphere usually corresponds to the superior temporal sulcus. The roof of the inferior horn is formed mainly by the tapetum of the corpus callosum, but also by the tail of the caudate nucleus and the stria terminalis, which extend forwards in the roof to terminate in the amygdala at the anterior end of the ventricle. The floor of the ventricle consists of the hippocampus medially and the collateral eminence, formed by the infolding of the collateral sulcus, laterally. The inferior part of the choroid fissure lies between the fimbria (a distinct bundle of efferent fibres that leaves the hippocampus) and the stria terminalis in the roof of the temporal horn (Fig. 16.5). The temporal extension of the choroid plexus fills this fissure and covers the outer surface of the hippocampus.

Fig. 16.2 Horizontal section of the cerebrum dissected to remove the roofs of the lateral ventricles.

Fig. 16.3 Coronal MRI scan, at the level of the anterior horn of the lateral ventricle.

Fig. 16.4 The lateral and third ventricles, coronal section.

Fig. 16.5 The inferior horn of the lateral ventricle, coronal section.

THIRD VENTRICLE

The third ventricle is a midline, slit-like cavity which is derived from the primitive forebrain vesicle (Figs 16.1, 16.4, 16.6, 16.7; see Fig. 15.8). The upper part of the lateral wall of the ventricle is formed by the medial surface of the anterior two-thirds of the thalamus, and the lower part is formed by the hypothalamus anteriorly and the subthalamus posteriorly. An indistinct hypothalamic sulcus extends horizontally on the ventricular wall between the interventricular foramen and the cerebral aqueduct, and marks the boundary between the thalamus and hypothalamus. Dorsally, the lateral wall is limited by a ridge covering the stria medullaris thalami. The lateral walls of the third ventricle are joined by an interthalamic adhesion, or massa intermedia, a band of grey matter which extends from one thalamus to the other.

Fig. 16.6 Sagittal MRI scan of the head.

Fig. 16.7 The third and fourth ventricles, sagittal hemisection of the brain.

Anteriorly, the third ventricle extends to the lamina terminalis (Fig. 16.7). This thin structure stretches from the optic chiasma to the rostrum of the corpus callosum and represents the rostral boundary of the embryonic neural tube. The lamina terminalis forms the roof of the small virtual cavity lying immediately below the ventricle called the cistern of the lamina terminalis. This is important because it contains the anterior communicating artery and aneurysm formation at this site may cause intraventricular haemorrhage through the thin membrane of the lamina terminalis. Above this, the anterior wall is formed by the diverging columns of the fornices and the transversely orientated anterior commissure, which crosses the midline. The anterior and posterior commissures are important neuroradiological landmarks. Prior to the introduction of modern imaging techniques, the fact that these commissures could be identified by ventriculography led to their use as reference markers for stereotaxic surgical procedures. This convention is now universal: the positions of the anterior and posterior commissures are used as the basic reference points for surgical atlases of brain anatomy. The narrow interventricular foramen is located immediately posterior to the column of the fornix and separates the fornix from the anterior nucleus of the thalamus.

There is a small, angular, optic recess at the base of the lamina terminalis, just dorsal to and extending into the optic chiasma. Behind it, the anterior part of the floor of the third ventricle is formed mainly by hypothalamic structures. Immediately behind the optic chiasma lies the thin infundibular recess, which extends into the pituitary stalk. Behind this recess, the tuber cinereum and the mammillary bodies form the floor of the ventricle.

The roof of the third ventricle is a thin ependymal layer that extends from its lateral walls to the choroid plexus, which spans the choroidal fissure (Fig. 16.4). Above the roof is the body of the fornix. The posterior boundary of the ventricle is marked by a suprapineal recess above the pineal gland, a pineal (epiphysial) recess, which extends into the pineal stalk, and by the posterior commissure. Below the commissure the ventricle is continuous with the cerebral aqueduct of the midbrain.

CEREBRAL AQUEDUCT

The cerebral aqueduct is a small tube, roughly circular in transverse section and 1–2 mm in diameter. It extends throughout the dorsal quarter of the midbrain in the midline and is surrounded by the periaqueductal (central) grey matter (Fig. 16.7; see Fig. 19.19). Rostrally, it commences immediately behind and below the posterior commissure, where it is continuous with the caudal aspect of the third ventricle. Caudally, it is continuous with the lumen of the fourth ventricle at the junction of the midbrain and pons. The superior and inferior colliculi are dorsal to the aqueduct and the midbrain tegmentum is ventral.

FOURTH VENTRICLE

The fourth ventricle lies between the brain stem and the cerebellum (Figs 16.1B, 16.6, 16.7, 16.8; see Fig. 19.3). Rostrally it is continuous with the cerebral aqueduct, and caudally with the central canal of the spinal cord. In sagittal section, the fourth ventricle has a characteristic triangular profile, and the apex of its tented roof protrudes into the inferior aspect of the cerebellum. The ventricle is at its widest at the level of the pontomedullary junction, where a lateral recess on both sides extends to the lateral border of the brain stem. At this point the lateral aperture of the fourth ventricle (foramen of Luschka) provides access to the subarachnoid space at the cerebellopontine angle; CSF flows through it into the lateral extension of the pontine cistern. Occasionally, a lateral recess may not open.

Fig. 16.8 The floor of the fourth ventricle, dorsal aspect of the brain stem.

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