d. Ventricles and Cerebrospinal Fluid.
(1) The ventricles (figures 3-20 and 3-21) are four communicating cavities
located within the brain. The lateral ventricles are located in the right and left cerebral
hemispheres. They communicate with the third ventricle through the interventricular
foramen (foramina of Monro). The third ventricle is a small passageway in the center of
the diencephalon and is connected with the fourth ventricle through the cerebral
aqueduct (aqueduct of Sylvius). The fourth ventricle is located anterior to the
cerebellum, posterior to the pons, and in the upper portion of the medulla oblongata. It
communicates with the subarachnoid space (between the arachnoid and pia mater
membranes) through three openings: two lateral foramina of Luschka and one medial
foramen of Magendie.
(2) The cerebrospinal fluid is a clear, viscid liquid formed in the choroid
plexuses, tortuous vascular masses on the roofs of the lateral and third ventricles. A
layer of epithelium separates the blood vessels of the choroid plexuses from the
ventricular activities. Cerebrospinal fluid is formed as an almost protein-free extract of
the blood. As the blood flows through the many choroid plexuses, water, traces of
protein, glucose, lymphocytes, and some hormones leave the blood plasma and enter
the ventricles as cerebrospinal fluid by active transport.
(3) Cerebrospinal fluid is enclosed in its own circulatory system within the
brain, passing from the right and left ventricles through the interventricular foramen of
Monro into the third ventricle and then by way of the cerebral aqueduct of Sylvius into
the fourth ventricle. From the fourth ventricle, it passes through the medial foramen of
Magendie and two lateral foramina into the subarachnoid space over the surface of the
brain. It may continue down the central canal of the spinal cord. In addition, the
cerebrospinal fluid moves caudally along the dorsal surface of the spinal cord and
cranially along its ventral surface in the subdural spaces. In the circulatory process, it
filters through small villi of arachnoid tissue into venous sinuses where it is returned to
the main bloodstream. Cerebrospinal fluid serves as a shock absorber for the brain and
spinal cord.
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