pia mater

pia ma·ter

noun \-ˈmāt-ər\

Definition of PIA MATER

: the delicate and highly vascular membrane of connective tissue investing the brain and spinal cord, lying internal to the arachnoid and dura mater, dipping down between the convolutions of the brain, and sending an ingrowth into the anterior fissure of the spinal cord—called also pia