Final answer:
The epineurium is the tough fibrous sheath that covers the whole nerve, providing protection and nourishment through blood vessels, while also organizing the nerve structure along with the perineurium and endoneurium.
Step-by-step explanation:
The tough fibrous sheath covering the whole nerve is called the epineurium. Nerves in the peripheral nervous system (PNS) are composed of more than just nervous tissue; they also include connective tissues and blood vessels. The outermost layer is the epineurium, which is a layer of fibrous connective tissue. Within the nerve, axons are grouped into fascicles, each encased by the perineurium, another layer of fibrous connective tissue. The individual axons themselves are surrounded by a loose connective tissue called the endoneurium. These protective layers help with the nourishment of the nerves through blood vessels and provide a structure similar to the connective tissue sheaths in muscles.