Complex sentence: Although anatomically and functionally connected to the brain, the pituitary gland is enveloped by the dura mater and sits outside the blood brain barrier separated from the subarachnoid space by the diaphragma sella therefore the arachnoid and thus cerebral spinal fluid cannot enter the sella turcica.
Split into simpler sentences: Although anatomically and functionally connected to the brain, the pituitary gland is enveloped by the dura mater and sits outside the blood brain barrier. It's separated from the subarachnoid space by the diaphragma sella, therefore the arachnoid and thus cerebral spinal fluid cannot enter the sella turcica.