What does "sheath" mean?

Sheath: Any sheathlike covering, organ, or part. Specifically: (a) (Bot.) The base of a leaf when sheathing or investing a stem or branch, as in grasses. (b) (Zoöl.) One of the elytra of an insect. Medullary sheath. (Anat.) See under Medullary. -- Primitive sheath. (Anat.) See Neurilemma. -- Sheath knife, a knife with a fixed blade, carried in a sheath. -- Sheath of Schwann. (Anat.) See Schwann's sheath.

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