Meninges of brain

The brain and spinal cord are enclosed by three membranous coverings for their protection known as the meninges of brain

• there are three Meninges of brain are present-
  1. Duramater
  2. Arachnoid
  3. Piamater

1. DURAMATER

• This is a dense, tough membrane of white fibrous tissue. It has cranial and spinal parts which enclose the brain and spinal cord, respectively and continuous with each other at the foramen magnum.
• The cranial duramateris not only acting as covering of the brain but also lines the inner surface of the cranium. It is composed of two layers, an outer periosteal layer which is adherent to the internal face of the cranial bones forming the internal periosteum and an inner meningeal layer which is in contact with the arachnoid. It forms the sheath around the nerves and leaving the cranium. These two layers are mostly adherent to each other except at places where they are separated to form the venous sinuses lined by endothelium. The internal surface of the dura is also lined by endothelium and is smooth. It forms the outer boundary of the subdural space that contains a small quantity of fluid resembling lymph.
• The duramater detaches a number of infoldings between the different sections of brain and form the falx cerebri, tentorium cerebelli and a thickening diaphragm sellae or pituitary fold.
• The falx cerebri is a sickle-shaped, vertical fold of duramater situated in the midline between the two cerebral hemispheres. It is attached above to the interparietal and interfrontal crests, behind to the internal occipital protruberance and in front to the crista galli of ethmoid. Its dorsal border is convex and here the two layers of the dura are separate and enclose the dorsal longitudinal sinus. Its ventral border is concave and lies above the corpus callosum. Here again the two layers enclose the ventral longitudinal sinus. Posteriorly it joins the tentorium cerebelli.
• The tentorium cerebelli is a crescentric, transverse fold between the cerebral hemispheres and cerebellum. It is attached above to the internal occipital protuberance and laterally to the petrosal crests. Its ventral border is concave, thin and free and forms an arch over the mid brain.
• The diaphragm sellae is a thick fold of duramater on the ventral aspect of the brain, which surrounds the pituitary gland and encloses the cavernous and intercavernous sinuses. It is perforated at the middle for the infundibulum and laterally for the exit of the emergent artery of the rete mirabile cerebri and the third cranial nerve on either side of the emergent artery.
• The hypophysis cerebrior pitutary gland is an oval reddish grey coloured body about the size of a large pea situated in the sella turicia of sphenoid bone, connected at the base of the brain with a stalk and enclosed by duramater. It consists of two lobes – anterior lobe and posterior lobe. The portion which are developed from the Rathke’s pouch are included under the term adenohypophysis. These are
  • pars distalis or pars anterior,
  • pars tuberalis and
  • pars intermedia. These parts constitute the major and important portion of the gland. In other domestic animals and birds no much appreciable difference is observed other than size.
• The spinal duramater starts from the foramen magnum and terminates at the middle of the sacrum. It is composed only of the meningeal layer and forms a very loose tube around the spinal cord. It is separated from the periosteum of the spinal canal by a considerable space called epidural space and is occupied by connective tissue and fat. The dura furnishes sheaths for cranial and spinal nerves.
• The subdural space is the potential space between the dura and arachnoid with small amount of fluid, which moistens the opposed endothelial surfaces.

2. ARACHNOID MEMBRANE

• It is a very thin, delicate, transparent membrane situated between the dura mater and piamater. The space between the arachnoid and piamater is known as subarachnoid space and it is filled with cerebrospinal fluid. The outer face of the arachnoid is lined by endothelium and forms the inner wall of the subdural space. Its internal face is also lined by endothelium and forms outer boundary of the subarachnoid space. The arachnoid furnishes sheaths for nerves only for a short distance. From the internal face of the membrane, several strands of connective tissue arise and join the piamater, thus converting the subarachnoid space into numerous intercommunicating cavities.
• The cerebral part does not dip into the sulci except the great longitudinal and transverse fissures. On the summits of the gyri, it is very closely adherent to the piamater. It bridges over the smaller sulci and here the subarachnoid space is divided into many intercommunicating cavities. In certain places, considerable space is enclosed between it and the piamater and they are the subarachnoid cisterns.
• They are chiefly four in number
  • The cisterna basalis is at the base of the brain and is extended from optic chiasma infront and continued posteriorly into the cisterna pontis.
  • The cisterna fossa lateralis is at the fossa lateralis.
  • The cisterna pontis is extending from the pituitary gland to the pons.
  • The cisterna magna is in the angle between the posterior parts of the inferior face of the cerebellum. It communicates with the fourth ventricle through a central and two lateral microscopic openings and behind with the subarachnoid space of the spinal cord.
• Along the dorsal border of the falx cerebri the arachnoid bears the bulbous excrescences the arachnoid granulations. These project into the dorsal longitudinal sinus.
• The spinal arachnoid is continuous with that of the cerebral part and it forms a relatively wide tube around the spinal cord so that the latter with the piamater is surrounded by a considerable quantity of cerebrospinal fluid. The subarachnoid space is traversed by fewer trabeculae than in the cranium. It is partially divided by the dorsal median septum and the two-denticulate ligament of the piamater.
• Cerebrospinal fluid: Tufts of capillaries inside the ventricles are covered by ependyma to form chorioidal plexus. The chorioid plexus of the lateral ventricles mainly forms CSF. CSF thus formed passes to the third ventricle through the  interventricular foramen of Monro, it flows from the third ventricle to fourth ventricle via  aqueduct of Sylvius. From fourth ventricle a small amount of fluid passes through the central canal of spinal cord and the major portion escapes to the subarachnoid space through the foramina Megendie and Luschka. The foramina Luschka are the two laterally placed apertures and foramen Megendie is the centrally placed aperture at the root of the fourth ventricle. The fluid is drained into the venous system mainly through arachnoid granulations.

3. PIAMATER

• It is a very delicate vascular membrane that closely invests the brain and the spinal cord and sends processes into the brain substance.
• The cerebral piamater accurately moulds itself on the brain by dipping itself into all the sulci and fissures. Its external face forms the inner boundary of the subarachnoid spaceand is lined by endothelium. From its deep face trabeculae are given off which dip into the substance of the brain. The smaller blood vessels ramify in the piamater forming plexuses.
• Folds of piamater extend into two of the fissures of the brain. One of these is the telachorioidea of the third ventricle that enters the transverse fissure and forms the roof of the third ventricle. Another fold passes into the space between the cerebellum and medulla and forms the telachorioidea of the fourth ventricle.
• The spinal piamater is thicker and denser. It sends a fold into the ventral median fissure of the spinal cord and also helps to form the dorsal median septum. Along the mid ventral line it forms a thickening the linea splendensin which runs the ventral spinal artery. On each side it gives off a strong longitudinal band, the denticulate ligament lying between the dorsal and ventral roots of the spinal nerves.