The biscuit outside spinal cord has three-layer velamen, most outside for stiff backbone film, it is among arachnoid, surface of clingy spinal cord is lining soft backbone film.

• 脊髓外面包有三层被膜，最外为硬脊膜，中间为蛛网膜，内层紧贴脊髓表面为软脊膜。