What is "medullary sheath"

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:

   1. (Bot.) The base of a leaf when sheathing or investing a stem or branch, as in grasses.

   2. (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.)

   1. Pertaining to, consisting of, or resembling, marrow or medulla.

   2. 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.

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