pia-arach·noid \ˈ ̷ ̷ ̷ ̷ ̷ ̷ˈ ̷ ̷ˌ ̷ ̷\noun orpi·a·rach·noid\ˌpīəˈrakˌnȯid\ Etymology:pia (I) : the pia mater and the arachnoid regarded as a single membrane investing the brain and spinal cord • pia-arach·noidal\| ̷ ̷ ̷ ̷ ̷ ̷| ̷ ̷ˌ ̷ ̷ ̷ ̷\adjective