Complex sentence: Pia mater is the thin, transluscent, mesh-like meningeal envelope loosely connected to the arachnoid layer and firmly adhered to the entire surface of the brain spanning down the spinal cord's neural tissue and running down into the fissures of the cerebral cortex.
Split into simpler sentences: Pia mater is the thin, transluscent, mesh-like meningeal envelope loosely connected to the arachnoid layer and firmly adhered to the entire surface of the brain. It spans down the spinal cord's neural tissue and runs down into the fissures of the cerebral cortex.