pia mater

(Lat., lit.=pious mother.] Anat .: A delicate, fibrous, and highly vascular membrane, immediately investing the brain and the spinal cord. ( Quain .) Hence, sometimes used for the brain itself, as in the example. "For here he comes, one of thy kin, has a most weak pia mater."- Shakesp .: Twelfth Night , i. 5.