and also into the two hemispheres as the longitudinal fissure. Fissures are the most prominent and invariable of the sulci.

The pia mater, the membrane surrounding the brain follows the surface of the brain into each sulcus, but the arachnoid mater stretches across all sizes of the sulci, except the longitudinal fissure where it follows the pia mater. Consequently the inner sides of almost all sulci are separated only