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subarachnoid space

The space between the arachnoidea and pia mater, traversed by delicate fibrous trabeculae and filled with cerebrospinal fluid. Since the pia mater immediately adheres to the surface of the brain and spinal cord, the space is greatly widened wherever the brain surface exhibits a deep depression (for example, between the cerebellum and medulla); such widenings are called cisternae. The large blood vessels supplying the brain and spinal cord lie in the subarachnoid space.

Synonym: cavum subarachnoideum, subarachnoid cavity.