Noun
The delicate outer sheath of a nerve fiber; the primitive sheath.
The perineurium.
Source: Webster's dictionaryCross section of a myelinated axon 1. Axon 2. Nucleus of Schwann Cell 3. Schwann Cell 4. Myelin Sheath 5. Neurilemma The main purpose of a myelin layer (or sheath) is to increase the speed at which impulses propagate along the myelinated fiber. Source: Internet