@pimms.value<EOL><INDENT>def midgray_surface(white_surface, pial_surface):<DEDENT>
midgray_coords = <NUM_LIT:0.5>*(white_surface.coordinates + pial_surface.coordinates)<EOL>return white_surface.copy(coordinates=midgray_coords)<EOL>
cortex.midgray_surface is the mesh representing the midgray surface half way between the
white and pial surfaces.