The arachnoid mater is the middle layer of the meninges.
 It is a thin, clear membrane that fits loosely over the pia mater.
 This leaves a space in between these two layers called the "subarachnoid space."
 ("Subarachnoid space" means "the space under the arachnoid.")
 Cerebrospinal fluid flows through the subarachnoid space.

One of the arachnoid layer's most important jobs is to cushion the brain.
 Like the pia mater, the arachnoid mater is made of cells that fluid cannot get through.
 Because fluid cannot get through either of these layers, the cerebrospinal fluid stays in the subarachnoid space and does not leak out.
 Having this layer of fluid helps protect the brain, like a car's airbag.
 Without this cushion, every time a person moved their head, their brain would hit the inside of the skull and would get hurt.

The arachnoid mater also has an important job in the blood-brain barrier.
 Blood runs through the dura mater, the last layer of the meninges.
 The arachnoid mater keeps blood from getting through into the cerebrospinal fluid.
 This is important, because if blood gets through the arachnoid mater into the cerebrospinal fluid, the brain and spinal cord can get irritated and infected.
 Like all parts of the body, the arachnoid mater needs blood and oxygen to survive.
 Small blood vessels bring these things to the arachnoid layer.
 However, the arachnoid layer prevents the blood from getting through to the cerebrospinal fluid.

The arachnoid mater is attached to the last layer of the meninges: the dura mater.
The arachnoid mater is the middle layer of the meninges.  It is a thin, clear membrane that fits loosely over the pia mater.  This leaves a space in between these two layers called the "subarachnoid space."  ("Subarachnoid space" means "the space under the arachnoid.")  Cerebrospinal fluid flows through the subarachnoid space.
One of the arachnoid layer's most important jobs is to cushion the brain.  Like the pia mater, the arachnoid mater is made of cells that fluid cannot get through.  Because fluid cannot get through either of these layers, the cerebrospinal fluid stays in the subarachnoid space and does not leak out.  Having this layer of fluid helps protect the brain, like a car's airbag.  Without this cushion, every time a person moved their head, their brain would hit the inside of the skull and would get hurt.
The arachnoid mater also has an important job in the blood-brain barrier.  Blood runs through the dura mater, the last layer of the meninges.  The arachnoid mater keeps blood from getting through into the cerebrospinal fluid.  This is important, because if blood gets through the arachnoid mater into the cerebrospinal fluid, the brain and spinal cord can get irritated and infected.  Like all parts of the body, the arachnoid mater needs blood and oxygen to survive.  Small blood vessels bring these things to the arachnoid layer.  However, the arachnoid layer prevents the blood from getting through to the cerebrospinal fluid.
The arachnoid mater is attached to the last layer of the meninges: the dura mater.
Meninges