Sheath (?), n. [OE. schethe, AS. scǣð, sceáð, scēð; akin to OS. skēðia, D. scheede, G. scheide, OHG. sceida, Sw. skida, Dan. skede, Icel. skeiðir, pl., and to E. shed, v.t., originally meaning, to separate, to part. See Shed.] 1. A case for the reception of a sword, hunting knife, or other long and slender instrument; a scabbard.
The dead knight's sword out of his sheath he drew. Spenser.
2. 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.