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3 definitions found
 for Medullary sheath
From 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.
        [1913 Webster]
              The dead knight's sword out of his sheath he drew.
        [1913 Webster]
     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.
            [1913 Webster]
     Medullary sheath. (Anat.) See under Medullary.
     Primitive sheath. (Anat.) See Neurilemma.
     Sheath knife, a knife with a fixed blade, carried in a
     Sheath of Schwann. (Anat.) See Schwann's sheath.
        [1913 Webster]

From 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
        (b) Pertaining to the medula oblongata.
            [1913 Webster]
     2. (Bot.) Filled with spongy pith; pithy.
        [1913 Webster]
     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.
        [1913 Webster]

From 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

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