Which layer surrounds entire nerves?

• A. Endoneurium
• B. Perineurium
• C. Dura mater
• D. Epineurium

Answer: (D)

The outermost protective layer of a peripheral nerve is the epineurium. It is a dense connective tissue sheath that surrounds the entire nerve, enclosing all the fascicles and providing a tough, elastic envelope that contains blood vessels and sometimes fat to nourish the nerve. Inside this outer wrap, individual axons are grouped into fascicles, each fascicle is wrapped by the perineurium, and within those fascicles the single nerve fibers are enveloped by the endoneurium. The dura mater, by contrast, is a protective membrane of the central nervous system, not a layer around peripheral nerves, so it isn’t the correct structure here.