In the peripheral nervous system (PNS) axons can be either myelinated or unmyelinated. Myelination means that the axon is insulated by surrounding it with layers of fatty membrane (myelin). These layers are continuous (like rolling up rope), but due to the way they are formed they often include small amounts of cytoplasm. These are the so called<unk><unk>  cleft (a.k.a.<unk> ,clefts of<unk><unk> , segments of<unk> , medullary segments), which subdivide the myelinated axon into irregular portions. 
They are histological evidence of the small amount of cytoplasm that remains in the inner layer of the myelin sheath created by Schwann cells wrapping tightly around a nerve.
Schmidt-Lanterman incisures
Horror vacui
Mees' lines
Microtia
Brechenmacher fibers
Neurotic excoriations
Interrod enamel
Olfactory foramina