The brain occupies the cranial cavity and is covered by membranes, fluid, and the bones of the skull. Although the brain’s various regions communicate and function together, the brain may be divided into distinct areas for ease of study (Fig. 6-1, Table 6):
* The cerebrum is the largest part of the brain. It is divided into right and left cerebral hemispheres by a deep groove called the longitudinal fissure (Fig. 6-2). Each hemisphere is further subdivided into lobes.
* The diencephalon is the area between the cerebral hemispheres and the brain stem. It includes the thalamus and the hypothalamus.
* The brain stem connects the cerebrum and diencephalon with the spinal cord. The superior portion of the brain stem is the midbrain. Inferior to the midbrain is the pons, followed by the medulla oblongata. The pons connects the midbrain with the medulla, whereas the medulla connects the brain with the spinal cord through a large opening in the base of the skull (foramen magnum).
* The cerebellum is located immediately below the posterior part of the cerebral hemispheres and is connected with the cerebrum, brain stem, and spinal cord by means of the pons. The word cerebellum means “little brain.”
Figure 6-1 Brain, sagittal section. Main divisions are shown.
Figure 6-2 External surface of the brain, superior view. The division into two hemispheres and into lobes is visible.
Table 6 Organization of the Brain
Protective Structures of the Brain and Spinal Cord
The meninges are three layers of connective tissue that surround both the brain and spinal cord to form a complete enclosure (Fig. 6-3). The outermost of these membranes, the dura mater, is the thickest and toughest of the meninges. (Mater is from the Latin meaning “mother,” referring to the protective function of the meninges; dura means “hard.”) Around the brain, the dura mater is in two layers, and the outer layer is fused to the bones of the cranium. In certain places, these two layers separate to provide venous channels, called dural sinuses, for the drainage of blood coming from the brain tissue.
The middle layer of the meninges is the arachnoid. This membrane is loosely attached to the deepest of the meninges by weblike fibers, allowing a space for the movement of cerebrospinal fluid (CSF) between the two membranes. (The arachnoid is named from the Latin word for spider because of its weblike appearance). The innermost layer around the brain, the pia mater, is attached to the nervous tissue of the brain and spinal cord and follows all the contours of these structures (see Fig. 6-3). It is made of a delicate connective tissue (pia meaning “tender” or “soft”). The pia mater holds blood vessels that supply nutrients and oxygen to the brain and spinal cord.
Figure 6-3 Frontal (coronal) section of the top of the head.
This fluid also carries nutrients to the cells and transports waste products from the cells. CSF flows freely through passageways in and around the brain and spinal cord and finally flows out into the subarachnoid space of the meninges. Much of the fluid then returns to the blood through projections called arachnoid villi in the dural sinuses (see Figs. 6-3 and 6-4).
Ventricles CSF forms in four spaces within the brain called ventricles (Fig. 6-5). A vascular network in each ventricle, the choroid plexus, forms CSF by filtration of the blood and by cellular secretion. The four ventricles that produce CSF extend somewhat irregularly into the various parts of the brain. The largest are the lateral ventricles in the two cerebral hemispheres. Their extensions into the lobes of the cerebrum are called horns. These paired ventricles communicate with a midline space, the third ventricle, by means of openings called foramina. The third ventricle is surrounded by the diencephalon. Continuing down from the third ventricle, a small canal, called the cerebral aqueduct, extends through the midbrain into the fourth ventricle, which is located between the brain stem and the cerebellum. This ventricle is continuous with the central canal of the spinal cord. In the roof of the fourth ventricle are three openings that allow the escape of CSF to the area that surrounds the brain and spinal cord.
Cerebrospinal fluid is a clear liquid that circulates in and around the brain and spinal cord (Fig. 6-4). The function of the CSF is to support nervous tissue and to cushion shocks that would otherwise injure these delicate structures.
Figure 6-5 Ventricles of the brain. Three views are shown.
Figure 6-4 Flow of cerebrospinal fluid (CSF). Black arrows show the flow of CSF from the choroid plexuses and back to the blood in dural sinuses; white arrows show the flow of blood. (The actual passageways through which the CSF flows are narrower than those shown here, which have been enlarged for visibility.)