VAN 121: Veterinary Gross Anatomy-II (2+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.