The Collaborative International Dictionary of English v.0.48:

Sheath \Sheath\, n. [OE. schethe, AS. sc[=ae][eth],
   sce['a][eth], sc[=e][eth]; akin to OS. sk[=e][eth]ia, D.
   scheede, G. scheide, OHG. sceida, Sw. skida, Dan. skede,
   Icel. skei[eth]ir, 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.
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            The dead knight's sword out of his sheath he drew.
                                                  --Spenser.
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   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) (Zool.) One of the elytra of an insect.
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   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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The Collaborative International Dictionary of English v.0.48:

Medullary \Med"ul*la*ry\, a. [L. medullaris, fr. medulla marrow:
   cf. F. m['e]dullaire.]
   1. (Anat.)
      (a) Pertaining to, consisting of, or resembling, marrow or
          medulla.
      (b) Pertaining to the medula oblongata.
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   2. (Bot.) Filled with spongy pith; pithy.
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   Medullary groove (Anat.), a groove, in the epiblast of the
      vertebrate blastoderm, the edges of which unite, making a
      tube (the medullary canal) from which the brain and spinal
      cord are developed.

   Medullary rays (Bot.), the rays of cellular tissue seen in
      a transverse section of exogenous wood, which pass from
      the pith to the bark.

   Medullary sheath (Anat.), the layer of white semifluid
      substance (myelin), between the primitive sheath and axis
      cylinder of a medullated nerve fiber.
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WordNet (r) 3.0 (2006):

medullary sheath
    n 1: a layer of myelin encasing (and insulating) the axons of
         medullated nerve fibers [syn: medullary sheath, myelin
         sheath]