The CNS contains an interconnecting series of chambers and channels that are derived from the lumen of the embryonic neural tube. In the spinal cord, this is represented by the vestigial and insignificant central canal. Within the brain, however, the enormous growth and distortion of the original tube-like structure is paralleled by the development of an elaborate system of ventricles (Figs 1.18, 6.1, 6.2).
Figure 6.1 Resin cast of the ventricular system. (A) Lateral view; (B) posterior view.
Figure 6.2 Median sagittal section of the brain showing the ventricular system.
Topographical anatomy of the ventricular system
In passing from the spinal cord to the brain stem, the central canal moves progressively more dorsal until, in the rostral (open) medulla, it opens out into a wide and shallow depression, the fourth ventricle(Fig. 6.3), which lies on the dorsal surface of the brain stem beneath the cerebellum. The fourth ventricle is rhomboid or diamond-shaped. On each side, a lateral recess extends towards the lateral margin of the brain stem and is in continuity, through a small lateral aperture (the foramen of Luschka), with the subarachnoid space of the cerebellopontine angle (Fig. 6.4). For the most part, the roof of the fourth ventricle is formed by the cerebellum. However, the roof of the caudal part consists of pia and ependyma, a central defect in which constitutes the median aperture of the fourth ventricle, or the foramen of Magendie, that provides communication with the cisterna magna (Fig. 6.5). The rostral part of the roof of the fourth ventricle is partly formed by the superior cerebellar peduncles on either side, the space between them being bridged by the thin superior medullary velum.
Figure 6.3 Dorsal aspect of brain stem illustrating the floor of the fourth ventricle.
Figure 6.4 Cerebellopontine angle. The point of continuity between the lateral recess of the fourth ventricle and the subarachnoid space is indicated by a small tuft of choroid plexus, which protrudes through the lateral aperture.
Figure 6.5 Posterior view of the brain. The cerebellum and brain stem have been slightly separated to show the median aperture of the fourth ventricle.
The fourth ventricle extends rostrally as far as the pontomesencephalic junction where it becomes continuous with the cerebral aqueduct, which passes throughout the length of the midbrain, beneath the inferior and superior colliculi. At the rostral margin of the midbrain, the cerebral aqueduct opens into the third ventricle, which is a narrow slit-like cavity whose lateral walls are formed by the thalamus and hypothalamus on either side (Fig. 6.2 and see Fig. 12.2). The roof of the ventricle is formed by pia-ependyma, which spans between two nerve fibre bundles, the striae medullaris thalami, which are situated along the dorsomedial border of the thalamus. In the rostral part of the third ventricle lies an aperture, the interventricular foramen or foramen of Monro, which is located between the column of the fornix and the anterior pole of the thalamus.
The interventricular foramen provides communication, on either side, with the extensive lateral ventricle located within the cerebral hemisphere (Fig. 6.6 and see Fig. 16.12). The lateral ventricle is approximately C-shaped. It consists of an anterior (frontal) horn, body, posterior (occipital) horn and inferior (temporal) horn. The anterior horn of the lateral ventricle is that part anterior to the interventricular foramen. Its lateral wall is the head of the caudate nucleus and its roof is the corpus callosum (see Figs 13.3–13.7Fig. 13.3Fig. 13.4Fig. 13.5Fig. 13.6Fig. 13.7). The medial wall is formed by the septum pellucidum. This thin sheet spans between the corpus callosum and fornix in the midline and separates the anterior horns of the two lateral ventricles. The body of the lateral ventricle extends behind the interventricular foramen, having as its floor the thalamus and tail of the caudate nucleus. More posteriorly, the small posterior horn extends towards the occipital pole but the principal course of the ventricle sweeps downwards and forwards to form the extensive inferior horn, which lies in the temporal lobe. In the floor of the inferior horn lies the hippocampus, while in its roof runs the much attenuated tail of the caudate nucleus (see Figs 13.9–13.12Fig. 13.9Fig. 13.10Fig. 13.11Fig. 13.12 and 16.12).
Figure 6.6 Superior aspect of a dissection of the cerebral hemispheres in which much of the corpus callosum has been removed to reveal the lumen of the lateral ventricles.
The ventricular system consists of the lateral ventricles, third ventricle, cerebral aqueduct and fourth ventricle.
The lateral ventricle is located within each cerebral hemisphere and is approximately C-shaped. It communicates, via the interventricular foramen, with the third ventricle.
The third ventricle is a midline, slit-like cavity. Its lateral walls consist of the thalamus and hypothalamus. Caudally, the third ventricle becomes continuous with the cerebral aqueduct.
The cerebral aqueduct extends throughout the midbrain, linking the third and fourth ventricles.
The fourth ventricle is located between the brain stem (pons and medulla) and the cerebellum. A median aperture and two lateral apertures communicate with the subarachnoid space surrounding the brain.
The ventricular system, together with the cranial and spinal subarachnoid spaces, contains cerebrospinal fluid (CSF; Fig. 6.7). This is produced by the choroid plexus, which is located in the lateral, third and fourth ventricles (Figs 6.2, 6.6 and see Fig. 16.12). The choroid plexus is formed by invagination of the vascular pia mater into the ventricular lumen, where it becomes highly convoluted, producing a sponge-like appearance. The choroid plexus enters the third and fourth ventricles through their roofs and the lateral ventricle through the choroid fissure, along the line of the fimbria/fornix (see Figs 16.8, 16.12).
Figure 6.7 Sagittal T2-weighted MR image of the brain demonstrating CSF within the ventricular system and subarachnoid space.
(Courtesy of Professor A Jackson, Wolfson Molecular Imaging Centre, University of Manchester, Manchester, UK.)
CSF is produced partly by an active secretory process and partly by passive diffusion. It is a colourless fluid containing little protein and few cells. The volume of CSF in the combined ventricular and subarachnoid spaces is approximately 150 mL. CSF is produced continuously, at a rate sufficient to fill these spaces several times each day. This means that an efficient mechanism is required for the circulation of CSF and its reabsorption (Figs 6.8-6.10).
Figure 6.8 The cerebral ventricular system and its relationship with the subarachnoid space. The circulation of cerebrospinal fluid is indicated by arrows.
Figure 6.9 Transverse section through the superior sagittal sinus showing arachnoid villi.
Figure 6.10 Superior aspect of the cerebral hemispheres showing arachnoid granulations on the right side. On the left side, the arachnoid mater has been removed.
Most CSF is produced by the choroid plexus of the lateral ventricle. From here it flows through the interventricular foramen into the third ventricle and thence, by way of the cerebral aqueduct, into the fourth ventricle. CSF leaves the ventricular system through the three apertures of the fourth ventricle and, thus, enters the subarachnoid space. Most CSF passes through the median aperture to enter the cisterna magna, located between the medulla and cerebellum. A lesser amount flows through the lateral apertures to enter the subarachnoid space in the region of the cerebellopontine angle. From these sites, the majority of CSF flows superiorly, round the cerebral hemispheres, to the sites of reabsorption. Within the subarachnoid space, CSF serves partially to cushion the brain from sudden movements of the head.
CSF is reabsorbed into the venous system by passing into the dural venous sinuses, principally the superior sagittal sinus. Along the sinuses are located numerous arachnoid villi, which consist of invaginations of arachnoid mater through the dural wall and into the lumen of the sinus (Fig. 6.9). Reabsorption occurs at these sites because the hydrostatic pressure in the subarachnoid space is higher than that in the lumen of the sinus and because of the greater colloid osmotic pressure of venous blood compared to CSF. With age, the arachnoid villi become hypertrophic to form arachnoid granulations (Fig. 6.10)
Obstruction of the flow of CSF within the ventricular system (e.g. by tumours) or the subarachnoid space (e.g. by adhesions following head injury or meningitis) leads to a rise in fluid pressure causing swelling of the ventricles (hydrocephalus). The clinical effects are similar to those of a brain tumour and consist of headaches, unsteadiness and mental impairment. Swelling of the optic discs (papilloedema) is seen on ophthalmoscopy. Decompression of the dilated ventricles is achieved by inserting a shunt connecting the ventricles to the jugular vein or the abdominal peritoneum.
Each ventricle contains choroid plexus, which secretes CSF.
CSF flows in the direction: lateral ventricle → third ventricle → cerebral aqueduct → fourth ventricle → subarachnoid space.
The combined ventricular system and subarachnoid space contains about 150 mL CSF, this volume being produced several times each day.
CSF is reabsorbed into the venous system through arachnoid villi, which project principally into the superior sagittal sinus.