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L ecture 15 Brain ابراهيم فاضل احمد****. د
The Meninges
Three protective membranes or meninges surround the brain in the skull: the dura mater, the arachnoid mater, and the pia mater
1- Dura Mater of the Brain
The dura mater is conventionally described as two layers: the endosteal layer and the meningeal layer. These are closely united except along certain lines, where they separate to form venous sinuses. The endosteal layer is nothing more than the ordinary periosteum covering the inner surface of the skull bones. It does not extend through the foramen magnum. The meningeal layer is the dura mater proper. It is a dense, strong, fibrous membrane covering the brain and is continuous through the foramen magnum with the dura mater of the spinal cord. The meningeal layer sends inward septa that divide the cranial cavity into freely communicating spaces. The function of these septa is to restrict the rotatory displacement of the brain. The falx cerebri is a sickle-shaped fold of dura mater that lies in the midline between the two cerebral hemispheres The tentorium cerebelli is a crescent-shaped fold of dura mater that roofs over the posterior cranial fossa and It covers the upper surface of the cerebellum and supports the occipital lobes of the cerebral hemispheres.
Sinuses of the brain
The venous sinuses of the cranial cavity are blood-filled spaces situated between the layers of the dura mater. They are lined by endothelium. Their walls are thick and composed of fibrous tissue; they have no muscular tissue. The sinuses have no valves. They receive tributaries from the brain, the diploë of the skull, the orbit, and the internal ear. 1- Superior sagittal sinus lies in the upper fixed border of the falx cerebri ,It runs backward and becomes continuous with the right transverse sinus. 2- Inferior sagittal sinus lies in the free lower margin of the falx cerebri. It runs backward and joins the great cerebral vein to form the straight sinus. 3- Straight sinus lies at the junction of the falx cerebri with the tentorium cerebelli. Formed by the union of the inferior sagittal sinus with the great cerebral vein, it drains into the left transverse sinus. 4- Right transverse sinus begins as a continuation of the superior sagittal sinus; 5- Left transverse sinus is usually a continuation of the straight sinus. Each sinus lies in the lateral attached margin of the tentorium cerebelli, and they end on each side by becoming the sigmoid sinus. 6- Sigmoid sinuses are a direct continuation of the transverse sinuses. Each sinus turns downward behind the mastoid antrum of the temporal bone and then leaves the skull through the jugular foramen to become the internal jugular vein. 7- Occipital sinus lies in the attached margin of the falx cerebelli. It communicates with the vertebral veins through the foramen magnum and the transverse sinuses. 8- Cavernous sinus lies on the lateral side of the body of the sphenoid bone. Anteriorly, the sinus receives the inferior ophthalmic vein and the central vein of the retina. The sinus drains posteriorly into the transverse sinus through the superior petrosal sinus. Intercavernous sinuses connect the two cavernous sinuses through the sella turcica.
Brain
Nervous system is the system which responds to the internal and external environments in order to maintain the internal environment and manipulate the external environment for survival and existence. Nervous system regulates all functions of the body.
Parts of Nervous System
1. Central nervous system (CNS): It consists of brain and spinal cord
Brain: It is also known as encephalon. It lies in the cranial cavity and continues as the spinal cord. It consists of following parts A. forebrain: It is further subdivided into cerebrum and diencephalon B. midbrain: It is made-up of cerebral peduncles. C. hindbrain: Hindbrain is made up of pons and medulla oblongata (ventrally) and cerebellum (dorsally). Spinal cord: It is also known as spinal medulla. It is the elongated part of central nervous system, which occupies the upper 2/3rd of the vertebral canal.
fissures conveniently subdivide the surface of each hemisphere into lobes. The lobes are named for the bones of the cranium under which they lie.
The frontal lobe is situated in front of the central sulcus and above the lateral sulcus. The parietal lobe is situated behind the central sulcus and above the lateral sulcus. The occipital lobe lies below the parietooccipital sulcus. Below the lateral sulcus is situated the temporal lobe. The precentral gyrus lies immediately anterior to the central sulcus and is known as the motor area. The large motor nerve cells in this area control voluntary movements on the opposite side of the body. In the motor area, the body is represented in an inverted position, with the nerve cells controlling the movements of the feet located in the upper part and those controlling the movements of the face and hands in the lower part The postcentral gyrus lies immediately posterior to the central sulcus and is known as the sensory area, the small nerve cells in this area receive and interpret sensations of pain, temperature, touch, and pressure from the opposite side of the body.
The superior temporal gyrus lies immediately below the lateral sulcus. The middle of this gyrus is known as the auditory area. Motor speech area, lies just above the lateral sulcus. It controls the movements employed in speech. It is dominant in the left hemisphere in right-handed persons and in the right hemisphere in left-handed persons. The visual area is situated on the posterior pole and medial aspect of the cerebral hemisphere.
2- Diencephalon
The diencephalon is almost completely hidden from the surface of the brain. It consists of a dorsal thalamus and a ventral hypothalamus. The thalamus is a large mass of gray matter that lies on either side of the third ventricle. The hypothalamus forms the lower part of the lateral wall and floor of the third ventricle.
Hindbrain
The pons is situated on the anterior surface of the cerebellum below the midbrain
and above the medulla oblongata; it is composed mainly of nerve fibers, which connect the two halves of the cerebellum. It also contains ascending and descending fibers connecting the forebrain, the midbrain, and the spinal cord.
The medulla oblongata is conical in shape and connects the pons above to the
spinal cord below.
The cerebellum lies within the posterior cranial fossa beneath the tentorium
cerebelli. It is situated posterior to the pons and the medulla oblongata. It consists of two hemispheres connected by a median portion, the vermis. The cerebellum is connected to the midbrain by the superior cerebellar peduncles, to the pons by the middle cerebellar peduncles, and to the medulla by the inferior cerebellar peduncles. The cerebellum plays an important role in the control of muscle tone and the coordination of muscle movement on the same side of the body.
Blood Supply of the Brain
Arteries of the Brain The brain is supplied by the two internal carotid and the two vertebral arteries. The four arteries anastomose on the inferior surface of the brain and form the circle of Willis (Discussed in the previous lecture) Veins of the Brain The veins of the brain have no muscular tissue in their thin walls, and they possess no valves. They emerge from the brain and drain into the cranial venous sinuses. Cerebral and cerebellar veins and veins of the brainstem are present. The great cerebral vein is formed by the union of the two internal cerebral veins and drains into the straight sinus.
