challenge: What is the composition of the inner surface of the dura mater?
  
  A. Interdigitating cell processes with no extracellular collagen
  B. Loosely arranged, fibroelastic layer of cells with significant extracellular spaces
  C. Flattened cells similar to those on the pia mater and arachnoid mater
  D. Dense fibrous tissue with a layer of collagen
  Answer: 