Abstract
Axons of the peripheral nervous system are always ensheathed, some with myelin and neurolemma sheath, others with the latter covering only. Nerve fibers possessing both coverings are spoken of as myelinated. Fibers of the peripheral nervous system surrounded by only the neurolemma sheath are called unmyelinated. It has been reported by Duncan (’34), after observations of the dorsal and ventral spinal nerve roots of the rat, cat, cow and pigeon stained with osmium tetroxide or pyridine silver, that there exists a critical axis cylinder diameter, between 1-2 m , above which the cylinder is myelinated and below which it is unmyelinated. Schwarzacher (154) from his studies on various human motor and sensory nerves, found that the relation of axon diameter to total diameter of myelinated fiber was constant and had a lower limiting value of 0.30. He concluded that myelinated fibers are a group quite distinct from unmyelinated fibers.