Central nervous system disorders

III./1.3.: Arachnoid

The outer layer of the leptomeninges was named after the trabecular fibers (arachnoideal trabeculae) of delicate connective tissue spanned between itself and the pia mater, which gives a spider web-like appearance to the subarachnoideal space. Formerly the term meninx serosa was used. The arachnoid membrane is separable from the dura mater, however a fine fibrous cohesion called the dura-arachnoida interface exists between them.

Unlike the pia mater the arachnoid invests the brain loosely, it is separated from the pia mater by the subarachnoid cavity, which is filled with cerebrospinal fluid (liquor cerebrospinalis). The subarachnoid space contains the superficial vessels of the central nervous system and till their dural penetration the cranial nerves. This space is not homogeneous, beside the mentioned delicate trabeculae it contains larger septa that divide its cavity into subarachnoideal cistern. Openings of variable size of these septa ensure the cerebrospinal fluid dynamics and the traverse of the vessels and nerves between the cisterns. Similarly the dura the arachnoid is also prolonged to form hornlike sheats for the nerves and vessels exiting the intradural space.

The arachnoid granulations (Pacchioni's granulations or pacchionian bodies – 1. drawing) are typical processes of the arachnoid. The bulk of the granulations lie along both sides of the superior sagittal sinus and drain the cerebrospinal fluid into its lateral lacunae, but they are also present along the sphenoparietal and cavernous sinuses. Rarely granulations may appear above the tegmental wall of the tympanic cavity, where beside the natural opening in the bone (dehiscence) rarely non-traumatic cerebrospinal fluid leak of the middle ear is observed.