[3][4] In sufficiently large nerves multiple fascicles, each with its blood supply and fatty tissue, may be bundled within yet another sheath, the epineurium.

The endoneurium runs longitudinally along the nerve fiber, but with discontinuities where septa pass inward from the innermost layer of the perineurium.

It contains fine bundles of fibrous connective tissue, primarily collagen, embedded in a matrix of ground substance.

[5] This structure serves to support capillary blood vessels, arranged so as to form a network of elongated meshes.

Similar structures occur around some nervous components elsewhere in the body, for example around the Schwann cells on the peripheral side of the transitional zone on the auditory nerve.