Source : Webster's Revised Unabridged Dictionary (1913)

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.

            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) (Zo["o]l.) 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}.

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.

   2. (Bot.) Filled with spongy pith; pithy.

   {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.

Source : WordNet®

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