The innermost membrane that covers the brain and spinal cord, located just inside the tougher protective layers.
From Medieval Latin 'pia mater' meaning 'tender mother', where 'pia' comes from Latin 'pius' meaning 'tender' or 'dutiful'. It was named this way because it's the most delicate of the three membranes protecting the brain. The poetic name reflects medieval anatomists' view of protecting structures.
The brain is wrapped in three membranes with beautiful Latin names: dura mater ('tough mother'), arachnoid ('spider-like'), and pia mater ('tender mother'). Medieval doctors used 'mother' metaphorically for protective layers, showing how science and poetry used to go together!
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