funnel-shaped interruptions of the major dense lines in the regular structure of the myelin sheath of nerve fibers, formerly interpreted as actual breaks in the sheath but shown by electron microscopy to correspond each to a strand of cytoplasm locally separating the two cytoplasmic aspects of the otherwise fused oligodendroglial (or, in peripheral nerves, Schwann cell) membranes composing the myelin sheath. SYN: Lanterman incisures, Schmidt-Lanterman clefts.