Sentence:  One screen in the theatre relays live colour pictures of Harkness and his colleague Tiernan Byrnes's progress, cutting and pushing through first the dura, then the arachnoid and finally the <WSD>pia</WSD>, the thin, spider's web-type membranes that cover the brain itself. 
Meaning of the word marked with <WSD> tags:  The pia mater, the innermost of the meninges that protect the brain and spinal cord