An abbreviation for 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). The large blood vessels supplying the brain and spinal cord lie in the
subarachnoid space.