Ventricles And Cerebrospinal Fluid

The brain contains four internal cavities called ventricles (Fig. 6). These cavities are filled with CSF, which is produced in the ventricles and serves to bathe the entire CNS. An understanding of the ventricular system is important for an appreciation of fluid and pressure dynamics in the brain, as well as for its anatomy.

The two largest ventricles are the lateral ventricles, one in each hemisphere, which underlie the frontal, parietal, occipital, and temporal lobes. These communicate with a single third ventricle, narrow and situated between the two thalami, via an opening in each lateral ventricle called the foramen of Monro. Finally, the tent-shaped fourth ventricle, just dorsal to the brain stem, is connected with the third ventricle by a small conduit in the midbrain called the cerebral aqueduct. The fourth ventricle, in turn, empties into a region of subarachnoid space called the cisterna magna through three apertures, the midline foramen of Magendie and the two lateral foramina of Luschka. More caudally, the CSF flowing from the ventricles travels down around the spinal cord to the lower end of the spinal canal and also circulates rostrally to the convexities of the brain, where it is eventually absorbed into the cerebral venous sinuses through structures known as arachnoid villi.

CSF is produced by the choroid plexus in all four ventricles, and the entire volume of the CSF in and around the CNS is about 140 ml. The volume of the ventricular system is actually rather small, about 25 ml. Because CSF is produced at the rate of about 450 ml/day, the entire CSF volume turns over more than three times daily. The CSF has an obvious supportive role in that it provides a buoyancy that keeps the brain from settling down upon rigid, bony surfaces. The CSF also takes part in regulating the chemical environment of brain neurons.

In clinical terms, the ventricular system and the CSF have many implications. For example, CSF analysis

Latera! ventricle

Latera! ventricle

ventricle

Figure 6 The position of the four ventricles—the two lateral ventricles, the third, and the fourth—within the brain. Reprinted with permission from Nolte, J. (1999). The Human Brain, 4th ed., p. 65. Mosby, New York.

ventricle

Figure 6 The position of the four ventricles—the two lateral ventricles, the third, and the fourth—within the brain. Reprinted with permission from Nolte, J. (1999). The Human Brain, 4th ed., p. 65. Mosby, New York.

after lumbar puncture is essential in the diagnosis of many neurologic disorders, such as meningitis and encephalitis. Moreover, enlargement of the ventricles due to an excess of CSF, as is seen in hydrocephalus or with certain mass lesions, can have major neurologic consequences.

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