pia mater

from WordNet (r) 3.0 (2006)

pia mater
    n 1: the highly vascular innermost of the 3 meninges

from The Collaborative International Dictionary of English v.0.48

Pia mater \Pi"a ma"ter\ [NL., fr. L. pia (fem. of pius tender,
   kind) + mater mother.] (Anat.)
   The delicate and highly vascular membrane immediately
   investing the brain and spinal cord.
   [1913 Webster]