The Arachnoid

Thin layer of connective tissue, on the internal and external surface is lined with a simple layer of flattened cells. The cobweb-like appearance is caused by numerous trabeculae connecting arachnoid and pia mater. The trabeculae are formed of loose connective tissue and the space bridged by these trabeculae (the space between the pia mater and arachnoid) is called the subarachnoid space. The subarachnoid space contains cerebrospinal fluid which slowly percolates through the trabeculae. When the pia closely follows the surface of the brain and the spinal cord, the arachnoid does not dip into the furrows on the brain surface and therefore the size of the subarachnoid space varies considerably. Large expansions of the subarachnoid space are termed as cisterns.