Porth's Essentials of Pathophysiology, 4e

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Nervous System

U N I T 1 0

Superior sagittal sinus Falx cerebri

Corpus callosum Anterior nucleus of thalamus

Inferior sagittal sinus

Cingulate gyrus

Fornix

Great cerebral vein (Galen)

Tentorium cerebelli

Transverse sinus

Olfactory bulb

Parahippocampal gyrus

Mammillary body

Temporal lobe

FIGURE 34-19. Cranial dura mater.The skull is open to show the falx cerebri and the right and left portions of the tentorium cerebelli, as well as some of the cranial venous sinuses.

Amygdala

Uncus

FIGURE 34-17. The limbic system includes the limbic cortex (cingulate gyrus, parahippocampal gyrus, uncus) and associated subcortical structures (mammillary body, amygdala).

the arachnoid mater is a continuous sheath of strong connective tissue, the dura mater (Latin for “tough mother”), which provides the major protection for the brain and spinal cord. The cranial dura often splits into two layers, with the outer layer serving as the perios- teum of the inner surface of the skull. The inner layer of the dura forms two major folds. The first, a longitudinal fold called the falx cerebri sepa- rates the cerebral hemispheres and fuses with a second transverse fold, the tentorium cerebelli (Fig. 34-19). The tentorium cerebelli separates the anterior and middle depression in the skull (cranial fossae), which contains the cerebral hemispheres, from the posterior fossa, found interiorly and containing the brain stem and cerebellum. Ventricular System and Cerebrospinal Fluid The ventricular system is a set of structures contain- ing cerebrospinal fluid (CSF) in the brain (Fig. 34-20). The system comprises four ventricles: right and left lateral ventricles (the first and second ventricles), third ventricle, and fourth ventricle. Cerebrospinal fluid is a clear, colorless ultrafiltrate of blood plasma, composed of 99% water with other constituents, making it close to the composition of the brain extracellular fluid. The total volume of CSF is 135 to 150 mL. The daily pro- duction is about 500 to 600 mL/day, so the CSF turns over about 4 times per day. The CSF provides a supporting and protective fluid in which the brain and spinal cord float, and it helps to main- tain a constant ionic environment that serves as a medium for diffusion of nutrients, electrolytes, and metabolic end products into the extracellular fluid surrounding CNS neu- rons and glia. Filling the ventricles, the CSF supports the mass of the brain. Because it fills the subarachnoid space surrounding the CNS, a physical force delivered to either the skull or spine is to some extent diffused and cushioned. The CSF is produced by tiny reddish masses of spe- cialized capillaries from the pia mater, called the choroid plexus , which projects into the ventricles. Once produced, the CSF flows freely through the ventricles (Fig. 34-20B). Three openings, or foramina, allow the CSF to pass into

Meninges Inside the skull and vertebral column, the brain and spinal cord are loosely suspended and protected by several connective tissue sheaths called the meninges (Fig. 34-18). The surfaces of the spinal cord, brain, and segmental nerves are covered with a delicate connec- tive tissue layer called the pia mater (Latin for “deli- cate mother”). The surface blood vessels and those that penetrate the brain and spinal cord are encased in this protective tissue layer. A second, very delicate, non- vascular, and waterproof layer, called the arachnoid mater , encloses the entire CNS. The arachnoid layer is named for its spider-web appearance. The CSF is con- tained in the subarachnoid space. Immediately outside

Superior sagittal sinus

Arachnoid villi

Skin

Subdural space

Periosteum

Bone

Subarachnoid space

Dura mater

Arachnoid

Pia mater

Falx cerebri

FIGURE 34-18. The cranial meninges. Arachnoid villi, shown within the superior sagittal sinus, are one site of cerebrospinal fluid absorption into the blood.