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The arachnoid mater is named after the Greek word arachne ("spider"), the suffix -oid ("in the image of"), and the Latin word mater ("mother"), because of the fine spider-web–like appearance of the delicate fibres of the arachnoid (arachnoid trabeculae) which extend down through the subarachnoid space and attach to the pia mater.
The middle element of the meninges is the arachnoid mater, or arachnoid membrane, so named because of its resemblance to a spider web. It cushions the central nervous system. This thin, transparent membrane is composed of fibrous tissue and, like the pia mater, has an outer layer of tightly packed flat cells, forming the arachnoid barrier. [8]
The arachnoid granulations act as one-way valves. Normally the pressure of the CSF is higher than that of the venous system, so CSF flows through the granulations into the blood. If the pressure is reversed for some reason, fluid will not pass back into the subarachnoid space. [6] The reason for this is not known.
The arachnoid trabeculae (AT) are delicate strands of connective tissue that loosely connect the two innermost layers of the meninges – the arachnoid mater and the pia mater. [ 1 ] [ 2 ] They are found within the subarachnoid space where cerebrospinal fluid is also found.
The outermost membrane, the dura mater, is a thick durable membrane, which is attached to both the arachnoid membrane and the skull. In bacterial meningitis, bacteria reach the meninges by one of two main routes: through the bloodstream (hematogenous spread) or through direct contact between the meninges and either the nasal cavity or the skin.
The pia firmly adheres to the surface of the brain and loosely connects to the arachnoid layer. [6] Because of this continuum, the layers are often referred to as the pia arachnoid or leptomeninges. A subarachnoid space exists between the arachnoid layer and the pia, into which the choroid plexus releases and maintains the cerebrospinal fluid ...
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The anatomy of spiders includes many characteristics shared with other arachnids. These characteristics include bodies divided into two tagmata (sections or segments), eight jointed legs, no wings or antennae, the presence of chelicerae and pedipalps , simple eyes , and an exoskeleton , which is periodically shed .