Ventricles and Cerebrospinal Fluid

A Lateral Ventricle Curves Through Each Cerebral Hemisphere

Each lateral ventricle follows a long C -shaped course through all the lobes of the cerebral hemisphere in which it resides. It is customarily divided into five parts ( Figs. 5.1 and 5.2 ): (1) an anterior (or frontal ) horn in the frontal lobe, anterior to the interventricular foramen; (2) a body in the frontal and parietal lobes, extending posteriorly to the region of the splenium of the corpus callosum; (3) a posterior (or occipital ) horn projecting posteriorly into the occipital lobe; (4) an inferior (or temporal ) horn curving inferiorly and anteriorly into the temporal lobe; and (5) an atrium, or trigone, the region near the splenium where the body and the posterior and inferior horns meet. The body, atrium, and inferior horn of each lateral ventricle represent the original C -shaped development of the ventricle; the anterior and posterior horns are extensions from this basic shape.

Various structures form the borders of the lateral ventricle in its course through the cerebral hemisphere; many of them can be seen easily in coronal sections (see Fig. 3.20, Fig. 3.21, Fig. 3.22, Fig. 3.23, Fig. 3.24, Fig. 3.25 ) or in brains dissected from above ( Fig. 5.3 ). The similarly C -shaped caudate nucleus (see Fig. 3.19 ) is a constant feature in sections through the ventricle. Its enlarged head forms the lateral wall of the anterior horn (see Fig. 3.20 ), its somewhat smaller body forms most of the lateral wall of the body of the ventricle (see Fig. 3.22 ), and its attenuated tail lies in the roof of the inferior horn (see Figs. 3.23 and 5.8C ). Proceeding posteriorly, as the caudate nucleus becomes smaller, the thalamus becomes larger and forms the floor of the body of the ventricle (compare Fig. 3.21, Fig. 3.22, Fig. 3.23 ). The corpus callosum and septum pellucidum give a good indication of the size and location of the anterior horn and body of the ventricle. The body of the corpus callosum forms the roof of these parts of the ventricle, and the genu of the corpus callosum curves inferiorly to form the anterior wall of the anterior horn. The septum pellucidum forms the medial wall of the body and anterior horn, and its termination near the splenium marks the site where the bodies of the ventricles diverge from the midline and begin to curve around into the inferior horns (compare Figs. 3.22 and 3.24 ).

The posterior horn is phylogenetically the most recently developed part of the lateral ventricle and is also the most variable in size, sometimes being rudimentary. There are a number of slight asymmetries between the cerebral hemispheres of the human brain, and the left posterior horn tends to be longer than the right, particularly in right-handed individuals. The two lateral ventricles are otherwise symmetrical.

The hippocampus forms most of the floor and medial wall of the inferior horn (see Fig. 5.8C ), which ends anteriorly at about the level of the uncus.

The Third Ventricle Is a Midline Cavity in the Diencephalon

The narrow, slit-shaped third ventricle occupies most of the midline region of the diencephalon (see Figs. 5.1 and 5.2 ), so its entire outline can be seen in a hemisected brain (see Fig. 3.16 ). It often looks like a misshapen doughnut in casts or reconstructions of the ventricular system (see Fig. 5.1 ). The hole in the doughnut corresponds to the interthalamic adhesion, which joins the thalami and crosses the ventricle in most human brains.

Anteriorly the third ventricle ends at the lamina terminalis, the remnant of the rostral neuropore. Much of the medial surface of the thalamus and hypothalamus forms the wall of the third ventricle, and part of the hypothalamus forms its floor. It has a thin, membranous roof containing choroid plexus (discussed in the next section). At the posterior end of the mammillary bodies, the third ventricle narrows fairly abruptly to become the cerebral aqueduct (of Sylvius), which traverses the midbrain. The interventricular foramen, in the anterior part of each wall of the third ventricle, is an important radiological landmark because its location can be visualized by several different methods and it bears a known anatomical relationship to a number of deep structures (e.g., it is at the anterior end of the thalamus). Blockage of one of the interventricular foramen is a common cause of obstructive or noncommunicating hydrocephalus (discussed later).

An outline of the third ventricle reveals four protrusions, called recesses ( Fig. 5.4 ), corresponding to structures that have evaginated from the diencephalon. Inferiorly the optic recess lies anterior to the optic chiasm at the base of the lamina terminalis; the infundibular recess lies immediately posterior to the chiasm. Superiorly the pineal recess invades the stalk of the pineal gland, and the suprapineal recess lies just anterior to this stalk.

The Fourth Ventricle Communicates With Subarachnoid Cisterns

The fourth ventricle is sandwiched between the cerebellum posteriorly and the pons and rostral medulla anteriorly (see Fig. 5.2 ). The floor is relatively flat, and because it narrows rostrally into the cerebral aqueduct and caudally into the central canal of the spinal cord, it is somewhat diamond shaped (see Fig. 11.3A ). For this reason, the floor is sometimes referred to as the rhomboid fossa. At the location where the lateral point of the diamond would be expected, the entire ventricle becomes a narrow tube that proceeds anteriorly and curves around the brainstem, ending adjacent to the flocculus of the cerebellum. This tubular prolongation is the lateral recess of the fourth ventricle (see Fig. 5.1 ). The rostral portion of the roof is the superior medullary velum, and the caudal portion is the inferior medullary velum. The superior medullary velum is a thin layer of white matter related to the cerebellum, whereas the inferior medullary velum is a membrane containing choroid plexus, similar to the roof of the third ventricle.

The lateral and third ventricles are nearly closed cavities, communicating only with other parts of the ventricular system. In contrast, there are three apertures in the fourth ventricle through which the ventricular system communicates freely with subarachnoid space. These are the unpaired median aperture (or foramen of Magendie ) and the two lateral apertures (or foramina of Luschka ) of the fourth ventricle (see Fig. 5.10 ). The median aperture is simply a hole in the inferior medullary velum ( Fig. 5.5 ); it is as though the caudal end of the membrane, where it should have closed off the ventricle at its junction with the central canal, was instead lifted up and attached to the inferior surface of the cerebellar vermis. The result is a funnel-shaped opening from the subarachnoid space (the cerebellomedullary cistern, or cisterna magna) into the ventricle. The inferior medullary velum also covers the lateral recess, and at the end of each recess is another opening in the velum, the lateral aperture.

The Ventricles Contain Only a Fraction of the CSF

The ventricles are smaller and more variable in size than may be expected. Although there is an average total of approximately 200 mL of CSF within and around the brain and spinal cord, only about 25 mL of this fluid is contained within the ventricles. The rest occupies subarachnoid space. The third and fourth ventricles together have a volume of only about 2 mL, and the volumes of the aqueduct and central canal are negligible, so the lateral ventricles contain nearly all the ventricular CSF. The total volume of 25 mL is only an average figure, and the ventricles of some apparently normal brains have been found to have total volumes of less than 10 mL or more than 30 mL (however, volumes greater than 30 mL are usually considered suspicious).